Hvac system with predictive free cooling control

ABSTRACT

A control system includes equipment and a controller. The equipment produces a resource by executing a first process when in a first state, and produces the resource by executing a second process when in a second state. The controller calculates a minimum second state operating time based on estimated cost savings resulting from operating in the second state relative to the first state. The minimum second state operating time is a minimum time amount that the equipment must operate in the second state for the estimated cost savings to offset cost of transitioning into the second state. The controller predicts whether the second process will be available for the minimum second state operating time during future time steps. The controller transitions the equipment from the first state to the second state in response to predicting that the second process will be available for the minimum second operating state time.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/925,466 filed Mar. 19, 2018, which is a continuation of U.S. patentapplication Ser. No. 15/411,878 filed Jan. 20, 2017, now U.S. Pat. No.9,982,903, the entire disclosures of which are incorporated by referenceherein.

BACKGROUND

The present disclosure relates generally to heating, ventilation, or airconditioning (HVAC) systems for a building. The present disclosurerelates more particularly to a HVAC system configured to operate in afree cooling state and a mechanical cooling state.

Free cooling is a cooling technique which uses low temperature outsideair to provide cooling for a system without requiring the use ofchillers. Free cooling can be used as an alternative to mechanicalcooling (e.g., vapor compression cooling) under certain weatherconditions. When free cooling is used, the chillers providing mechanicalcooling can be deactivated and cooling towers used to provide freecooling.

In traditional free cooling systems, free cooling is used whenever theoutdoor wet bulb air temperature is below a minimum temperature requiredfor free cooling. However, the traditional approach does not take intoaccount the economic cost associated with transitioning between freecooling and mechanical cooling. For example, switching between amechanical cooling state and a free cooling state may incur an economiccost. It would be desirable to provide a HVAC system which useseconomically optimal control to transition between a mechanical coolingstate and a free cooling state.

SUMMARY

One implementation of the present disclosure is a control system,according to some embodiments. The control system includes equipment anda controller, according to some embodiments. The equipment is configuredto produce a resource by executing a first process when operating in afirst state, and produce the resource by executing a second process whenoperating in a second state, according to some embodiments. Thecontroller is configured to calculate a minimum second state operatingtime based on an estimated cost savings resulting from operating in thesecond state relative to operating in the first state, according to someembodiments. In some embodiments, the minimum second state operatingtime is a minimum amount of time that the equipment is required tooperate in the second state for the estimated cost savings to offset acost of transitioning into the second state. In some embodiments, thecontroller is configured to predict whether the second process will beavailable for at least the minimum second state operating time duringfuture time steps. In some embodiments, the controller is configured totransition the equipment from operating in the first state to operatingin the second state in response to a prediction that the second processwill be available for at least the minimum second operating state time.

In some embodiments, the controller is configured to calculate theminimum second state operating time by identifying the cost incurred asa result of transitioning the equipment into the second state,estimating the cost savings resulting from operating the equipment toexecute the second process in the second state relative to operating theequipment to execute the first process in the first state as a functionof an amount of time the equipment operates in the second state, anddetermining the minimum amount of time the equipment is required tooperate in the second state for the cost savings to be greater than orequal to the cost incurred.

In some embodiments, the controller is configured to estimate the costsavings by determining an amount of energy savings resulting fromoperating the equipment in the second state relative to operating theequipment in the first state, identifying a cost per unit energy foreach of the future time steps during which the equipment will operate inthe second, and calculating the cost savings by multiplying the amountof energy savings by the cost per unit energy.

In some embodiments, the cost incurred includes at least one of aneconomic cost of equipment degradation and an increase in electricitycost resulting from stopping and restarting the equipment.

In some embodiments, predicting whether the second process will beavailable for at least the minimum second state operating time duringthe future time steps includes predicting a value of a variable thatindicates an availability of the second process during each of thefuture time steps, and, for each time step of the future time steps,determining whether the second process will be available during the timestep by comparing the value of the variable to a threshold value.

In some embodiments, the controller is configured to predict whether thesecond process will be unavailable for at least a minimum first stateoperating time. In some embodiments, the controller is furtherconfigured to transition the equipment from operating in the secondstate to operating in the first state in response to a prediction thatthe second process will be unavailable for at least the minimum firststate operating time.

In some embodiments, the controller is configured to determine whetherthe second process is available at a current time. In some embodiments,the controller is further configured to transition the equipment fromoperating in the second state to operating in a third state during whichthe resource is not produced in response to a determination that thesecond process is unavailable at the current time.

In some embodiments, the controller is configured to transition theequipment from operating in the second state to operating in the thirdstate in response to a determination that the second process isunavailable at the current time and predicted to become available withina predetermined amount of time.

Another implementation of the present disclosure is a controller forequipment, according to some embodiments. The controller includes one ormore processors and one or more non-transitory computer-readable storagemedia communicably coupled to the one or more processors, according tosome embodiments. In some embodiments, the one or more non-transitorycomputer-readable storage have instructions stored thereon that, whenexecuted by the one or more processors, cause the one or more processorsto operate the equipment to execute a first process in a first state toproduce a resource, and operate the equipment to execute a secondprocess in a second state to produce the resource. In some embodiments,the instructions are configured to cause the one or more processors tocalculate a minimum second state operating time based on an estimatedcost savings resulting from operating in the second state relative tooperating in the first state, wherein the minimum second state operatingtime is a minimum amount of time that the equipment is required tooperate in the second state for the estimated cost savings to offset acost of transitioning into the second state. In some embodiments, theinstructions are configured to cause the one or more processors topredict whether the second process will be available for at least theminimum second state operating time during future time steps, andtransition the equipment from operating in the first state to operatingin the second state in response to a prediction that the second processwill be available for at least the minimum second state operating time.

In some embodiments, the instructions cause the one or more processorsto calculate the minimum second state operating time by identifying thecost incurred as a result of transitioning the equipment into the secondstate, and estimating the cost savings resulting from operating theequipment in the second state relative to operating the equipment in thefirst state as a function of an amount of time the equipment operates inthe second state. In some embodiments, the instructions cause the one ormore processors to determine the minimum amount of time the equipment isrequired to operate in the second state for the cost savings to begreater than or equal to the cost incurred.

In some embodiments, the instructions cause the one or more processorsto estimate the cost savings by determining an amount of energy savingsresulting from operating the equipment in the second state relative tooperating the equipment in the first state, and identifying a cost perunit energy for each of the future time steps during which the equipmentwill operate in the second state. In some embodiments, the instructionscause the one or more processors to calculate the cost savings bymultiplying the amount of energy savings by the cost per unit energy.

In some embodiments, the cost incurred includes at least one of aneconomic cost of equipment degradation and an increase in electricitycost resulting from stopping and restarting the equipment.

In some embodiments, predicting whether the second process will beavailable for at least the minimum second state operating time duringthe future time steps includes predicting a value of a variable thatidentifies an availability of the second process for the future timesteps, and, for each time step of the future time steps, determiningwhether the second process will be available during the time step bycomparing the value of the variable to a threshold value.

In some embodiments, the instructions cause the one or more processorsto predict whether the second process will be unavailable for at least aminimum first state operating time, and transition the equipment fromoperating in second state to operating in the first state in response toa prediction that the second process will be unavailable for at leastthe minimum first state operating time.

In some embodiments, the instructions cause the one or more processorsto determine whether the second process is available at a current time,and transition the equipment from operating in the second state tooperating in a third state during which the resource is not produced inresponse to a determination that the second process is unavailable atthe current time.

One implementation of the present disclosure is a method for controllingequipment, according to some embodiments. In some embodiments, themethod includes producing a resource by operating equipment to execute afirst process when operating in a first state. In some embodiments, themethod further includes producing the resource by operating equipment toexecute a second process when operating equipment in a second state. Insome embodiments, the method further includes calculating a minimumsecond state operating time based on an estimated cost savings resultingfrom operating in the second state relative to operating in the firststate. In some embodiments, the minimum second state operating time is aminimum amount of time that the equipment is required to operate in thesecond state for the estimated cost savings to offset a cost oftransitioning into the second state. In some embodiments, the methodfurther includes predicting whether the second process will be availablefor at least the minimum second state operating time during future timesteps, and transitioning the equipment from operating in the first stateto operating in the second state in response to a prediction that thesecond process will be available for at least the minimum secondoperating state time.

In some embodiments, calculating the minimum second state operating timeincludes identifying the cost incurred as a result of transitioning theequipment into the second state, and estimating the cost savingsresulting from operating the equipment to execute the second process inthe second state relative to operating the equipment to execute thefirst process in the first state as a function of an amount of time theequipment operates in the second state. In some embodiments, calculatingthe minimum second state operating time further includes determining theminimum amount of time the equipment is required to operate in thesecond state for the cost savings to be greater than or equal to thecost incurred.

In some embodiments, estimating the cost savings includes determining anamount of energy savings resulting from operating the equipment in thesecond state relative to operating the equipment in the first state,identifying a cost per unit energy for each of the future time stepsduring which the equipment will operate in the second, and calculatingthe cost savings by multiplying the amount of energy savings by the costper unit energy.

In some embodiments, predicting whether the second process is availablefor at least the second state operating time during the future timesteps includes predicting a value of a variable that indicates anavailability of the second process during each of the future time steps,and, for each time step of the future time steps, determining whetherthe second process will be available during the time step by comparingthe value of the variable to a threshold value.

In some embodiments, the method further includes predicting whether thesecond process will be unavailable for at least a minimum first stateoperating time, and transitioning the equipment from operating in thesecond state to operating in the first state in response to a predictionthat the second process will be unavailable for at least the minimumfirst state operating time.

Those skilled in the art will appreciate that the summary isillustrative only and is not intended to be in any way limiting. Otheraspects, inventive features, and advantages of the devices and/orprocesses described herein, as defined solely by the claims, will becomeapparent in the detailed description set forth herein and taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments will become more fully understood from the followingdetailed description, taken in conjunction with the accompanyingdrawings, wherein like reference numerals refer to like elements, inwhich:

FIG. 1 is a drawing of a building equipped with a heating, ventilation,or air conditioning (HVAC) system, according to an exemplary embodiment;

FIG. 2 is a drawing of a waterside system which can be used incombination with the HVAC system of FIG. 1, according to an exemplaryembodiment;

FIG. 3 is a drawing of an airside system which can be used incombination with the HVAC system of FIG. 1, according to an exemplaryembodiment;

FIG. 4 is a block diagram of a building management system which can beused to monitor and control the building and HVAC system of FIG. 1,according to an exemplary embodiment;

FIG. 5 is a block diagram of another building management system whichcan be used to monitor and control the building and HVAC system of FIG.1, according to an exemplary embodiment;

FIG. 6 is a block diagram of a HVAC system configured to operate in amechanical cooling state and a free cooling state, according to anexemplary embodiment;

FIG. 7 is a block diagram illustrating operation of the HVAC system ofFIG. 6 in the mechanical cooling state, according to an exemplaryembodiment;

FIG. 8 is a block diagram illustrating operation of the HVAC system ofFIG. 6 in the free cooling state, according to an exemplary embodiment;

FIG. 9 is a block diagram illustrating a portion of the HVAC system ofFIG. 6 in greater detail, according to an exemplary embodiment;

FIG. 10 is a state transition diagram illustrating the state transitionsand transition conditions used by the HVAC system of FIG. 6, accordingto an exemplary embodiment; and

FIG. 11 is a flowchart of a process for operating the HVAC system ofFIG. 6 in the mechanical cooling state and the free cooling state isshown, according to an exemplary embodiment.

DETAILED DESCRIPTION Overview

Referring generally to the FIGURES, a heating, ventilation, or airconditioning (HVAC) system with predictive free cooling control andcomponents thereof are shown, according to various exemplaryembodiments. The HVAC system can operate in both a mechanical coolingstate and a free cooling state. The HVAC system can transition betweenthe mechanical cooling state and free cooling state to provideeconomically optimal cooling for a cooling load. In some embodiments,the HVAC system includes a controller which can evaluate statetransition conditions and transition between operating states based on aresult of the evaluation.

In traditional free cooling systems, free cooling is typically usedwhenever the outdoor wet bulb air temperature is below a minimumtemperature required for free cooling. However, the traditional approachdoes not take into account the economic cost associated withtransitioning between operating states. For example, switching between amechanical cooling state and a free cooling state may incur an economiccost. The economic cost may result from increased electricityconsumption when a chiller is starting-up, increased equipmentdegradation resulting from switching chillers on/off, inefficientchiller operation during start-up, electricity required to operatevalves, and/or any other economic costs which are incurred as a resultof the state transition.

To make free cooling economically viable, the energy and cost savingsachieved by free cooling should be sufficient to overcome the costincurred as a result of transitioning between the mechanical coolingstate and the free cooling state. Advantageously, HVAC system describedherein can determine whether the use of free cooling would beeconomically viable by weighing the cost savings achieved by freecooling against the economic cost of performing the state transition.For example, free cooling may be economically viable only if the freecooling lasts for a minimum amount of time. The controller can predicthow long the use of free cooling would last as well as the energysavings which would be achieved by the use of free cooling during thepredicted free cooling period. The controller can weigh the predictedenergy savings against the cost of performing the state transition todetermine whether to transition into the free cooling state.

In some embodiments, the controller is configured to predict the outsideair temperature {circumflex over (T)}_(OA) (e.g., predicted outside airwet bulb temperature) for each of a plurality of time steps into thefuture. The controller can predict the outside air temperature{circumflex over (T)}_(OA) using measurements from sensors and/orweather forecasts from a weather service. When operating in themechanical cooling state, the controller can determine whether thepredicted outside air temperature {circumflex over (T)}_(OA) will bebelow a free cooling temperature threshold T_(FC) for a predeterminedamount of time in the future. The controller can transition from themechanical cooling state to the free cooling state in response to adetermination that the predicted outside air temperature {circumflexover (T)}_(OA) will remain below the free cooling temperature thresholdT_(FC) for the predetermined amount of time.

In some embodiments, the free cooling temperature threshold T_(FC) is amaximum outdoor air wet bulb temperature at which free cooling ispossible or economically viable. The predetermined amount of time may bea minimum amount of time t_(min,FC) which free cooling must last inorder to justify the economic cost of transitioning into the freecooling state. If the predicted outside air temperature {circumflex over(T)}_(OA) will not stay below the temperature threshold T_(FC) for thepredetermined amount of time t_(min,FC), the controller can remain inthe mechanical cooling state, even if the current outside airtemperature T_(OA) is below the temperature threshold T_(FC). Thisprevents the HVAC system from transitioning into the free cooling stateif the amount of time spent in the free cooling state and thecorresponding energy savings are insufficient to overcome the costincurred as a result of the state transition. Additional features andadvantages of the HVAC system are described in detail below.

Building HVAC Systems and Building Management Systems

Referring now to FIGS. 1-5, several building management systems (BMS)and HVAC systems in which the systems and methods of the presentdisclosure can be implemented are shown, according to some embodiments.In brief overview, FIG. 1 shows a building 10 equipped with a HVACsystem 100. FIG. 2 is a block diagram of a waterside system 200 whichcan be used to serve building 10. FIG. 3 is a block diagram of anairside system 300 which can be used to serve building 10. FIG. 4 is ablock diagram of a BMS which can be used to monitor and control building10. FIG. 5 is a block diagram of another BMS which can be used tomonitor and control building 10.

Building and HVAC System

Referring particularly to FIG. 1, a perspective view of a building 10 isshown. Building 10 is served by a BMS. A BMS is, in general, a system ofdevices configured to control, monitor, and manage equipment in oraround a building or building area. A BMS can include, for example, aHVAC system, a security system, a lighting system, a fire alertingsystem, any other system that is capable of managing building functionsor devices, or any combination thereof.

The BMS that serves building 10 includes a HVAC system 100. HVAC system100 can include a plurality of HVAC devices (e.g., heaters, chillers,air handling units, pumps, fans, thermal energy storage, etc.)configured to provide heating, cooling, ventilation, or other servicesfor building 10. For example, HVAC system 100 is shown to include awaterside system 120 and an airside system 130. Waterside system 120 mayprovide a heated or chilled fluid to an air handling unit of airsidesystem 130. Airside system 130 may use the heated or chilled fluid toheat or cool an airflow provided to building 10. An exemplary watersidesystem and airside system which can be used in HVAC system 100 aredescribed in greater detail with reference to FIGS. 2-3.

HVAC system 100 is shown to include a chiller 102, a boiler 104, and arooftop air handling unit (AHU) 106. Waterside system 120 may use boiler104 and chiller 102 to heat or cool a working fluid (e.g., water,glycol, etc.) and may circulate the working fluid to AHU 106. In variousembodiments, the HVAC devices of waterside system 120 can be located inor around building 10 (as shown in FIG. 1) or at an offsite locationsuch as a central plant (e.g., a chiller plant, a steam plant, a heatplant, etc.). The working fluid can be heated in boiler 104 or cooled inchiller 102, depending on whether heating or cooling is required inbuilding 10. Boiler 104 may add heat to the circulated fluid, forexample, by burning a combustible material (e.g., natural gas) or usingan electric heating element. Chiller 102 may place the circulated fluidin a heat exchange relationship with another fluid (e.g., a refrigerant)in a heat exchanger (e.g., an evaporator) to absorb heat from thecirculated fluid. The working fluid from chiller 102 and/or boiler 104can be transported to AHU 106 via piping 108.

In some embodiments, HVAC system 100 uses free cooling to cool theworking fluid. For example, HVAC system 100 can include one or morecooling towers or heat exchangers which transfer heat from the workingfluid to outside air. Free cooling can be used as an alternative orsupplement to mechanical cooling via chiller 102 when the temperature ofthe outside air is below a threshold temperature. HVAC system 100 canswitch between free cooling and mechanical cooling based on the currenttemperature of the outside air and/or the predicted future temperatureof the outside air. An example of a free cooling system which can beused in HVAC system 100 is described in greater detail with reference toFIG. 6.

AHU 106 may place the working fluid in a heat exchange relationship withan airflow passing through AHU 106 (e.g., via one or more stages ofcooling coils and/or heating coils). The airflow can be, for example,outside air, return air from within building 10, or a combination ofboth. AHU 106 may transfer heat between the airflow and the workingfluid to provide heating or cooling for the airflow. For example, AHU106 can include one or more fans or blowers configured to pass theairflow over or through a heat exchanger containing the working fluid.The working fluid may then return to chiller 102 or boiler 104 viapiping 110.

Airside system 130 may deliver the airflow supplied by AHU 106 (i.e.,the supply airflow) to building 10 via air supply ducts 112 and mayprovide return air from building 10 to AHU 106 via air return ducts 114.In some embodiments, airside system 130 includes multiple variable airvolume (VAV) units 116. For example, airside system 130 is shown toinclude a separate VAV unit 116 on each floor or zone of building 10.VAV units 116 can include dampers or other flow control elements thatcan be operated to control an amount of the supply airflow provided toindividual zones of building 10. In other embodiments, airside system130 delivers the supply airflow into one or more zones of building 10(e.g., via supply ducts 112) without using intermediate VAV units 116 orother flow control elements. AHU 106 can include various sensors (e.g.,temperature sensors, pressure sensors, etc.) configured to measureattributes of the supply airflow. AHU 106 may receive input from sensorslocated within AHU 106 and/or within the building zone and may adjustthe flow rate, temperature, or other attributes of the supply airflowthrough AHU 106 to achieve setpoint conditions for the building zone.

Waterside System

Referring now to FIG. 2, a block diagram of a waterside system 200 isshown, according to some embodiments. In various embodiments, watersidesystem 200 may supplement or replace waterside system 120 in HVAC system100 or can be implemented separate from HVAC system 100. Whenimplemented in HVAC system 100, waterside system 200 can include asubset of the HVAC devices in HVAC system 100 (e.g., boiler 104, chiller102, pumps, valves, etc.) and may operate to supply a heated or chilledfluid to AHU 106. The HVAC devices of waterside system 200 can belocated within building 10 (e.g., as components of waterside system 120)or at an offsite location such as a central plant.

In FIG. 2, waterside system 200 is shown as a central plant having aplurality of subplants 202-212. Subplants 202-212 are shown to include aheater subplant 202, a heat recovery chiller subplant 204, a chillersubplant 206, a cooling tower subplant 208, a hot thermal energy storage(TES) subplant 210, and a cold thermal energy storage (TES) subplant212. Subplants 202-212 consume resources (e.g., water, natural gas,electricity, etc.) from utilities to serve thermal energy loads (e.g.,hot water, cold water, heating, cooling, etc.) of a building or campus.For example, heater subplant 202 can be configured to heat water in ahot water loop 214 that circulates the hot water between heater subplant202 and building 10. Chiller subplant 206 can be configured to chillwater in a cold water loop 216 that circulates the cold water betweenchiller subplant 206 building 10. Heat recovery chiller subplant 204 canbe configured to transfer heat from cold water loop 216 to hot waterloop 214 to provide additional heating for the hot water and additionalcooling for the cold water. Condenser water loop 218 may absorb heatfrom the cold water in chiller subplant 206 and reject the absorbed heatin cooling tower subplant 208 or transfer the absorbed heat to hot waterloop 214. Hot TES subplant 210 and cold TES subplant 212 may store hotand cold thermal energy, respectively, for subsequent use.

Hot water loop 214 and cold water loop 216 may deliver the heated and/orchilled water to air handlers located on the rooftop of building 10(e.g., AHU 106) or to individual floors or zones of building 10 (e.g.,VAV units 116). The air handlers push air past heat exchangers (e.g.,heating coils or cooling coils) through which the water flows to provideheating or cooling for the air. The heated or cooled air can bedelivered to individual zones of building 10 to serve thermal energyloads of building 10. The water then returns to subplants 202-212 toreceive further heating or cooling.

Although subplants 202-212 are shown and described as heating andcooling water for circulation to a building, it is understood that anyother type of working fluid (e.g., glycol, CO2, etc.) can be used inplace of or in addition to water to serve thermal energy loads. In otherembodiments, subplants 202-212 may provide heating and/or coolingdirectly to the building or campus without requiring an intermediateheat transfer fluid. These and other variations to waterside system 200are within the teachings of the present disclosure.

Each of subplants 202-212 can include a variety of equipment configuredto facilitate the functions of the subplant. For example, heatersubplant 202 is shown to include a plurality of heating elements 220(e.g., boilers, electric heaters, etc.) configured to add heat to thehot water in hot water loop 214. Heater subplant 202 is also shown toinclude several pumps 222 and 224 configured to circulate the hot waterin hot water loop 214 and to control the flow rate of the hot waterthrough individual heating elements 220. Chiller subplant 206 is shownto include a plurality of chillers 232 configured to remove heat fromthe cold water in cold water loop 216. Chiller subplant 206 is alsoshown to include several pumps 234 and 236 configured to circulate thecold water in cold water loop 216 and to control the flow rate of thecold water through individual chillers 232.

Heat recovery chiller subplant 204 is shown to include a plurality ofheat recovery heat exchangers 226 (e.g., refrigeration circuits)configured to transfer heat from cold water loop 216 to hot water loop214. Heat recovery chiller subplant 204 is also shown to include severalpumps 228 and 230 configured to circulate the hot water and/or coldwater through heat recovery heat exchangers 226 and to control the flowrate of the water through individual heat recovery heat exchangers 226.Cooling tower subplant 208 is shown to include a plurality of coolingtowers 238 configured to remove heat from the condenser water incondenser water loop 218. Cooling tower subplant 208 is also shown toinclude several pumps 240 configured to circulate the condenser water incondenser water loop 218 and to control the flow rate of the condenserwater through individual cooling towers 238.

In some embodiments, waterside system 200 uses free cooling to cool thewater in cold water loop 216. For example, the water returning from thebuilding in cold water loop 216 can be delivered to cooling towersubplant 208 and through cooling towers 238. Cooling towers 238 canremove heat from the water in cold water loop 216 (e.g., by transferringthe heat to outside air) to provide free cooling for the water in coldwater loop 216. In some embodiments, waterside system 200 switchesbetween free cooling with cooling tower subplant 208 and mechanicalcooling with chiller subplant 208 based on the current temperature ofthe outside air and/or the predicted future temperature of the outsideair. An example of a free cooling system which can be used in watersidesystem 200 is described in greater detail with reference to FIG. 6.

Hot TES subplant 210 is shown to include a hot TES tank 242 configuredto store the hot water for later use. Hot TES subplant 210 may alsoinclude one or more pumps or valves configured to control the flow rateof the hot water into or out of hot TES tank 242. Cold TES subplant 212is shown to include cold TES tanks 244 configured to store the coldwater for later use. Cold TES subplant 212 may also include one or morepumps or valves configured to control the flow rate of the cold waterinto or out of cold TES tanks 244.

In some embodiments, one or more of the pumps in waterside system 200(e.g., pumps 222, 224, 228, 230, 234, 236, and/or 240) or pipelines inwaterside system 200 include an isolation valve associated therewith.Isolation valves can be integrated with the pumps or positioned upstreamor downstream of the pumps to control the fluid flows in watersidesystem 200. In various embodiments, waterside system 200 can includemore, fewer, or different types of devices and/or subplants based on theparticular configuration of waterside system 200 and the types of loadsserved by waterside system 200.

Airside System

Referring now to FIG. 3, a block diagram of an airside system 300 isshown, according to some embodiments. In various embodiments, airsidesystem 300 may supplement or replace airside system 130 in HVAC system100 or can be implemented separate from HVAC system 100. Whenimplemented in HVAC system 100, airside system 300 can include a subsetof the HVAC devices in HVAC system 100 (e.g., AHU 106, VAV units 116,ducts 112-114, fans, dampers, etc.) and can be located in or aroundbuilding 10. Airside system 300 may operate to heat or cool an airflowprovided to building 10 using a heated or chilled fluid provided bywaterside system 200.

In FIG. 3, airside system 300 is shown to include an economizer-type airhandling unit (AHU) 302. Economizer-type AHUs vary the amount of outsideair and return air used by the air handling unit for heating or cooling.For example, AHU 302 may receive return air 304 from building zone 306via return air duct 308 and may deliver supply air 310 to building zone306 via supply air duct 312. In some embodiments, AHU 302 is a rooftopunit located on the roof of building 10 (e.g., AHU 106 as shown inFIG. 1) or otherwise positioned to receive both return air 304 andoutside air 314. AHU 302 can be configured to operate exhaust air damper316, mixing damper 318, and outside air damper 320 to control an amountof outside air 314 and return air 304 that combine to form supply air310. Any return air 304 that does not pass through mixing damper 318 canbe exhausted from AHU 302 through exhaust damper 316 as exhaust air 322.

Each of dampers 316-320 can be operated by an actuator. For example,exhaust air damper 316 can be operated by actuator 324, mixing damper318 can be operated by actuator 326, and outside air damper 320 can beoperated by actuator 328. Actuators 324-328 may communicate with an AHUcontroller 330 via a communications link 332. Actuators 324-328 mayreceive control signals from AHU controller 330 and may provide feedbacksignals to AHU controller 330. Feedback signals can include, forexample, an indication of a current actuator or damper position, anamount of torque or force exerted by the actuator, diagnosticinformation (e.g., results of diagnostic tests performed by actuators324-328), status information, commissioning information, configurationsettings, calibration data, and/or other types of information or datathat can be collected, stored, or used by actuators 324-328. AHUcontroller 330 can be an economizer controller configured to use one ormore control algorithms (e.g., state-based algorithms, extremum seekingcontrol (ESC) algorithms, proportional-integral (PI) control algorithms,proportional-integral-derivative (PID) control algorithms, modelpredictive control (MPC) algorithms, feedback control algorithms, etc.)to control actuators 324-328.

Still referring to FIG. 3, AHU 302 is shown to include a cooling coil334, a heating coil 336, and a fan 338 positioned within supply air duct312. Fan 338 can be configured to force supply air 310 through coolingcoil 334 and/or heating coil 336 and provide supply air 310 to buildingzone 306. AHU controller 330 may communicate with fan 338 viacommunications link 340 to control a flow rate of supply air 310. Insome embodiments, AHU controller 330 controls an amount of heating orcooling applied to supply air 310 by modulating a speed of fan 338.

Cooling coil 334 may receive a chilled fluid from waterside system 200(e.g., from cold water loop 216) via piping 342 and may return thechilled fluid to waterside system 200 via piping 344. Valve 346 can bepositioned along piping 342 or piping 344 to control a flow rate of thechilled fluid through cooling coil 334. In some embodiments, coolingcoil 334 includes multiple stages of cooling coils that can beindependently activated and deactivated (e.g., by AHU controller 330, byBMS controller 366, etc.) to modulate an amount of cooling applied tosupply air 310.

Heating coil 336 may receive a heated fluid from waterside system 200(e.g., from hot water loop 214) via piping 348 and may return the heatedfluid to waterside system 200 via piping 350. Valve 352 can bepositioned along piping 348 or piping 350 to control a flow rate of theheated fluid through heating coil 336. In some embodiments, heating coil336 includes multiple stages of heating coils that can be independentlyactivated and deactivated (e.g., by AHU controller 330, by BMScontroller 366, etc.) to modulate an amount of heating applied to supplyair 310.

Each of valves 346 and 352 can be controlled by an actuator. Forexample, valve 346 can be controlled by actuator 354 and valve 352 canbe controlled by actuator 356. Actuators 354-356 may communicate withAHU controller 330 via communications links 358-360. Actuators 354-356may receive control signals from AHU controller 330 and may providefeedback signals to controller 330. In some embodiments, AHU controller330 receives a measurement of the supply air temperature from atemperature sensor 362 positioned in supply air duct 312 (e.g.,downstream of cooling coil 334 and/or heating coil 336). AHU controller330 may also receive a measurement of the temperature of building zone306 from a temperature sensor 364 located in building zone 306.

In some embodiments, AHU controller 330 operates valves 346 and 352 viaactuators 354-356 to modulate an amount of heating or cooling providedto supply air 310 (e.g., to achieve a setpoint temperature for supplyair 310 or to maintain the temperature of supply air 310 within asetpoint temperature range). The positions of valves 346 and 352 affectthe amount of heating or cooling provided to supply air 310 by coolingcoil 334 or heating coil 336 and may correlate with the amount of energyconsumed to achieve a desired supply air temperature. AHU 330 maycontrol the temperature of supply air 310 and/or building zone 306 byactivating or deactivating coils 334-336, adjusting a speed of fan 338,or a combination of both.

In some embodiments, AHU controller 330 uses free cooling to cool supplyair 310. AHU controller 330 can switch between free cooling andmechanical cooling by operating outside air damper 320 and cooling coil334. For example, AHU controller 330 can deactivate cooling coil 334 andopen outside air damper 320 to allow outside air 314 to enter supply airduct 312 in response to a determination that free cooling iseconomically optimal. AHU controller 330 can determine whether freecooling is economically optimal based on the temperature of outside air314 and/or the predicted future temperature of outside air 314. Forexample, AHU controller 330 can determine whether the temperature ofoutside air 314 is predicted to be below a threshold temperature for apredetermined amount of time. An example of free cooling switching logicwhich can be used by AHU controller 330 is described in greater detailwith reference to FIG. 10.

Still referring to FIG. 3, airside system 300 is shown to include abuilding management system (BMS) controller 366 and a client device 368.BMS controller 366 can include one or more computer systems (e.g.,servers, supervisory controllers, subsystem controllers, etc.) thatserve as system level controllers, application or data servers, headnodes, or master controllers for airside system 300, waterside system200, HVAC system 100, and/or other controllable systems that servebuilding 10. BMS controller 366 may communicate with multiple downstreambuilding systems or subsystems (e.g., HVAC system 100, a securitysystem, a lighting system, waterside system 200, etc.) via acommunications link 370 according to like or disparate protocols (e.g.,LON, BACnet, etc.). In various embodiments, AHU controller 330 and BMScontroller 366 can be separate (as shown in FIG. 3) or integrated. In anintegrated implementation, AHU controller 330 can be a software moduleconfigured for execution by a processor of BMS controller 366.

In some embodiments, AHU controller 330 receives information from BMScontroller 366 (e.g., commands, setpoints, operating boundaries, etc.)and provides information to BMS controller 366 (e.g., temperaturemeasurements, valve or actuator positions, operating statuses,diagnostics, etc.). For example, AHU controller 330 may provide BMScontroller 366 with temperature measurements from temperature sensors362-364, equipment on/off states, equipment operating capacities, and/orany other information that can be used by BMS controller 366 to monitoror control a variable state or condition within building zone 306.

Client device 368 can include one or more human-machine interfaces orclient interfaces (e.g., graphical user interfaces, reportinginterfaces, text-based computer interfaces, client-facing web services,web servers that provide pages to web clients, etc.) for controlling,viewing, or otherwise interacting with HVAC system 100, its subsystems,and/or devices. Client device 368 can be a computer workstation, aclient terminal, a remote or local interface, or any other type of userinterface device. Client device 368 can be a stationary terminal or amobile device. For example, client device 368 can be a desktop computer,a computer server with a user interface, a laptop computer, a tablet, asmartphone, a PDA, or any other type of mobile or non-mobile device.Client device 368 may communicate with BMS controller 366 and/or AHUcontroller 330 via communications link 372.

Building Management Systems

Referring now to FIG. 4, a block diagram of a building management system(BMS) 400 is shown, according to some embodiments. BMS 400 can beimplemented in building 10 to automatically monitor and control variousbuilding functions. BMS 400 is shown to include BMS controller 366 and aplurality of building subsystems 428. Building subsystems 428 are shownto include a building electrical subsystem 434, an informationcommunication technology (ICT) subsystem 436, a security subsystem 438,a HVAC subsystem 440, a lighting subsystem 442, a lift/escalatorssubsystem 432, and a fire safety subsystem 430. In various embodiments,building subsystems 428 can include fewer, additional, or alternativesubsystems. For example, building subsystems 428 may also oralternatively include a refrigeration subsystem, an advertising orsignage subsystem, a cooking subsystem, a vending subsystem, a printeror copy service subsystem, or any other type of building subsystem thatuses controllable equipment and/or sensors to monitor or controlbuilding 10. In some embodiments, building subsystems 428 includewaterside system 200 and/or airside system 300, as described withreference to FIGS. 2-3.

Each of building subsystems 428 can include any number of devices,controllers, and connections for completing its individual functions andcontrol activities. HVAC subsystem 440 can include many of the samecomponents as HVAC system 100, as described with reference to FIGS. 1-3.For example, HVAC subsystem 440 can include a chiller, a boiler, anynumber of air handling units, economizers, field controllers,supervisory controllers, actuators, temperature sensors, and otherdevices for controlling the temperature, humidity, airflow, or othervariable conditions within building 10. Lighting subsystem 442 caninclude any number of light fixtures, ballasts, lighting sensors,dimmers, or other devices configured to controllably adjust the amountof light provided to a building space. Security subsystem 438 caninclude occupancy sensors, video surveillance cameras, digital videorecorders, video processing servers, intrusion detection devices, accesscontrol devices and servers, or other security-related devices.

Still referring to FIG. 4, BMS controller 366 is shown to include acommunications interface 407 and a BMS interface 409. Interface 407 mayfacilitate communications between BMS controller 366 and externalapplications (e.g., monitoring and reporting applications 422,enterprise control applications 426, remote systems and applications444, applications residing on client devices 448, etc.) for allowinguser control, monitoring, and adjustment to BMS controller 366 and/orsubsystems 428. Interface 407 may also facilitate communications betweenBMS controller 366 and client devices 448. BMS interface 409 mayfacilitate communications between BMS controller 366 and buildingsubsystems 428 (e.g., HVAC, lighting security, lifts, powerdistribution, business, etc.).

Interfaces 407, 409 can be or include wired or wireless communicationsinterfaces (e.g., jacks, antennas, transmitters, receivers,transceivers, wire terminals, etc.) for conducting data communicationswith building subsystems 428 or other external systems or devices. Invarious embodiments, communications via interfaces 407, 409 can bedirect (e.g., local wired or wireless communications) or via acommunications network 446 (e.g., a WAN, the Internet, a cellularnetwork, etc.). For example, interfaces 407, 409 can include an Ethernetcard and port for sending and receiving data via an Ethernet-basedcommunications link or network. In another example, interfaces 407, 409can include a Wi-Fi transceiver for communicating via a wirelesscommunications network. In another example, one or both of interfaces407, 409 can include cellular or mobile phone communicationstransceivers. In one embodiment, communications interface 407 is a powerline communications interface and BMS interface 409 is an Ethernetinterface. In other embodiments, both communications interface 407 andBMS interface 409 are Ethernet interfaces or are the same Ethernetinterface.

Still referring to FIG. 4, BMS controller 366 is shown to include aprocessing circuit 404 including a processor 406 and memory 408.Processing circuit 404 can be communicably connected to BMS interface409 and/or communications interface 407 such that processing circuit 404and the various components thereof can send and receive data viainterfaces 407, 409. Processor 406 can be implemented as a generalpurpose processor, an application specific integrated circuit (ASIC),one or more field programmable gate arrays (FPGAs), a group ofprocessing components, or other suitable electronic processingcomponents.

Memory 408 (e.g., memory, memory unit, storage device, etc.) can includeone or more devices (e.g., RAM, ROM, Flash memory, hard disk storage,etc.) for storing data and/or computer code for completing orfacilitating the various processes, layers and modules described in thepresent application. Memory 408 can be or include volatile memory ornon-volatile memory. Memory 408 can include database components, objectcode components, script components, or any other type of informationstructure for supporting the various activities and informationstructures described in the present application. According to someembodiments, memory 408 is communicably connected to processor 406 viaprocessing circuit 404 and includes computer code for executing (e.g.,by processing circuit 404 and/or processor 406) one or more processesdescribed herein.

In some embodiments, BMS controller 366 is implemented within a singlecomputer (e.g., one server, one housing, etc.). In various otherembodiments BMS controller 366 can be distributed across multipleservers or computers (e.g., that can exist in distributed locations).Further, while FIG. 4 shows applications 422 and 426 as existing outsideof BMS controller 366, in some embodiments, applications 422 and 426 canbe hosted within BMS controller 366 (e.g., within memory 408).

Still referring to FIG. 4, memory 408 is shown to include an enterpriseintegration layer 410, an automated measurement and validation (AM&V)layer 412, a demand response (DR) layer 414, a fault detection anddiagnostics (FDD) layer 416, an integrated control layer 418, and abuilding subsystem integration later 420. Layers 410-420 can beconfigured to receive inputs from building subsystems 428 and other datasources, determine optimal control actions for building subsystems 428based on the inputs, generate control signals based on the optimalcontrol actions, and provide the generated control signals to buildingsubsystems 428. The following paragraphs describe some of the generalfunctions performed by each of layers 410-420 in BMS 400.

Enterprise integration layer 410 can be configured to serve clients orlocal applications with information and services to support a variety ofenterprise-level applications. For example, enterprise controlapplications 426 can be configured to provide subsystem-spanning controlto a graphical user interface (GUI) or to any number of enterprise-levelbusiness applications (e.g., accounting systems, user identificationsystems, etc.). Enterprise control applications 426 may also oralternatively be configured to provide configuration GUIs forconfiguring BMS controller 366. In yet other embodiments, enterprisecontrol applications 426 can work with layers 410-420 to optimizebuilding performance (e.g., efficiency, energy use, comfort, or safety)based on inputs received at interface 407 and/or BMS interface 409.

Building subsystem integration layer 420 can be configured to managecommunications between BMS controller 366 and building subsystems 428.For example, building subsystem integration layer 420 may receive sensordata and input signals from building subsystems 428 and provide outputdata and control signals to building subsystems 428. Building subsystemintegration layer 420 may also be configured to manage communicationsbetween building subsystems 428. Building subsystem integration layer420 translate communications (e.g., sensor data, input signals, outputsignals, etc.) across a plurality of multi-vendor/multi-protocolsystems.

Demand response layer 414 can be configured to optimize resource usage(e.g., electricity use, natural gas use, water use, etc.) and/or themonetary cost of such resource usage in response to satisfy the demandof building 10. The optimization can be based on time-of-use prices,curtailment signals, energy availability, or other data received fromutility providers, distributed energy generation systems 424, fromenergy storage 427 (e.g., hot TES 242, cold TES 244, etc.), or fromother sources. Demand response layer 414 may receive inputs from otherlayers of BMS controller 366 (e.g., building subsystem integration layer420, integrated control layer 418, etc.). The inputs received from otherlayers can include environmental or sensor inputs such as temperature,carbon dioxide levels, relative humidity levels, air quality sensoroutputs, occupancy sensor outputs, room schedules, and the like. Theinputs may also include inputs such as electrical use (e.g., expressedin kWh), thermal load measurements, pricing information, projectedpricing, smoothed pricing, curtailment signals from utilities, and thelike.

According to some embodiments, demand response layer 414 includescontrol logic for responding to the data and signals it receives. Theseresponses can include communicating with the control algorithms inintegrated control layer 418, changing control strategies, changingsetpoints, or activating/deactivating building equipment or subsystemsin a controlled manner. Demand response layer 414 may also includecontrol logic configured to determine when to utilize stored energy. Forexample, demand response layer 414 may determine to begin using energyfrom energy storage 427 just prior to the beginning of a peak use hour.

In some embodiments, demand response layer 414 includes a control moduleconfigured to actively initiate control actions (e.g., automaticallychanging setpoints) which minimize energy costs based on one or moreinputs representative of or based on demand (e.g., price, a curtailmentsignal, a demand level, etc.). In some embodiments, demand responselayer 414 uses equipment models to determine an optimal set of controlactions. The equipment models can include, for example, thermodynamicmodels describing the inputs, outputs, and/or functions performed byvarious sets of building equipment. Equipment models may representcollections of building equipment (e.g., subplants, chiller arrays,etc.) or individual devices (e.g., individual chillers, heaters, pumps,etc.).

Demand response layer 414 may further include or draw upon one or moredemand response policy definitions (e.g., databases, XML files, etc.).The policy definitions can be edited or adjusted by a user (e.g., via agraphical user interface) so that the control actions initiated inresponse to demand inputs can be tailored for the user's application,desired comfort level, particular building equipment, or based on otherconcerns. For example, the demand response policy definitions canspecify which equipment can be turned on or off in response toparticular demand inputs, how long a system or piece of equipment shouldbe turned off, what setpoints can be changed, what the allowable setpoint adjustment range is, how long to hold a high demand setpointbefore returning to a normally scheduled setpoint, how close to approachcapacity limits, which equipment modes to utilize, the energy transferrates (e.g., the maximum rate, an alarm rate, other rate boundaryinformation, etc.) into and out of energy storage devices (e.g., thermalstorage tanks, battery banks, etc.), and when to dispatch on-sitegeneration of energy (e.g., via fuel cells, a motor generator set,etc.).

Integrated control layer 418 can be configured to use the data input oroutput of building subsystem integration layer 420 and/or demandresponse later 414 to make control decisions. Due to the subsystemintegration provided by building subsystem integration layer 420,integrated control layer 418 can integrate control activities of thesubsystems 428 such that the subsystems 428 behave as a singleintegrated supersystem. In some embodiments, integrated control layer418 includes control logic that uses inputs and outputs from a pluralityof building subsystems to provide greater comfort and energy savingsrelative to the comfort and energy savings that separate subsystemscould provide alone. For example, integrated control layer 418 can beconfigured to use an input from a first subsystem to make anenergy-saving control decision for a second subsystem. Results of thesedecisions can be communicated back to building subsystem integrationlayer 420.

Integrated control layer 418 is shown to be logically below demandresponse layer 414. Integrated control layer 418 can be configured toenhance the effectiveness of demand response layer 414 by enablingbuilding subsystems 428 and their respective control loops to becontrolled in coordination with demand response layer 414. Thisconfiguration may advantageously reduce disruptive demand responsebehavior relative to conventional systems. For example, integratedcontrol layer 418 can be configured to assure that a demandresponse-driven upward adjustment to the setpoint for chilled watertemperature (or another component that directly or indirectly affectstemperature) does not result in an increase in fan energy (or otherenergy used to cool a space) that would result in greater total buildingenergy use than was saved at the chiller.

Integrated control layer 418 can be configured to provide feedback todemand response layer 414 so that demand response layer 414 checks thatconstraints (e.g., temperature, lighting levels, etc.) are properlymaintained even while demanded load shedding is in progress. Theconstraints may also include setpoint or sensed boundaries relating tosafety, equipment operating limits and performance, comfort, fire codes,electrical codes, energy codes, and the like. Integrated control layer418 is also logically below fault detection and diagnostics layer 416and automated measurement and validation layer 412. Integrated controllayer 418 can be configured to provide calculated inputs (e.g.,aggregations) to these higher levels based on outputs from more than onebuilding subsystem.

Automated measurement and validation (AM&V) layer 412 can be configuredto verify that control strategies commanded by integrated control layer418 or demand response layer 414 are working properly (e.g., using dataaggregated by AM&V layer 412, integrated control layer 418, buildingsubsystem integration layer 420, FDD layer 416, or otherwise). Thecalculations made by AM&V layer 412 can be based on building systemenergy models and/or equipment models for individual BMS devices orsubsystems. For example, AM&V layer 412 may compare a model-predictedoutput with an actual output from building subsystems 428 to determinean accuracy of the model.

Fault detection and diagnostics (FDD) layer 416 can be configured toprovide on-going fault detection for building subsystems 428, buildingsubsystem devices (i.e., building equipment), and control algorithmsused by demand response layer 414 and integrated control layer 418. FDDlayer 416 may receive data inputs from integrated control layer 418,directly from one or more building subsystems or devices, or fromanother data source. FDD layer 416 may automatically diagnose andrespond to detected faults. The responses to detected or diagnosedfaults can include providing an alert message to a user, a maintenancescheduling system, or a control algorithm configured to attempt torepair the fault or to work-around the fault.

FDD layer 416 can be configured to output a specific identification ofthe faulty component or cause of the fault (e.g., loose damper linkage)using detailed subsystem inputs available at building subsystemintegration layer 420. In other exemplary embodiments, FDD layer 416 isconfigured to provide “fault” events to integrated control layer 418which executes control strategies and policies in response to thereceived fault events. According to some embodiments, FDD layer 416 (ora policy executed by an integrated control engine or business rulesengine) may shut-down systems or direct control activities around faultydevices or systems to reduce energy waste, extend equipment life, orassure proper control response.

FDD layer 416 can be configured to store or access a variety ofdifferent system data stores (or data points for live data). FDD layer416 may use some content of the data stores to identify faults at theequipment level (e.g., specific chiller, specific AHU, specific terminalunit, etc.) and other content to identify faults at component orsubsystem levels. For example, building subsystems 428 may generatetemporal (i.e., time-series) data indicating the performance of BMS 400and the various components thereof. The data generated by buildingsubsystems 428 can include measured or calculated values that exhibitstatistical characteristics and provide information about how thecorresponding system or process (e.g., a temperature control process, aflow control process, etc.) is performing in terms of error from itssetpoint. These processes can be examined by FDD layer 416 to exposewhen the system begins to degrade in performance and alert a user torepair the fault before it becomes more severe.

Referring now to FIG. 5, a block diagram of another building managementsystem (BMS) 500 is shown, according to some embodiments. BMS 500 can beused to monitor and control the devices of HVAC system 100, watersidesystem 200, airside system 300, building subsystems 428, as well asother types of BMS devices (e.g., lighting equipment, securityequipment, etc.) and/or HVAC equipment.

BMS 500 provides a system architecture that facilitates automaticequipment discovery and equipment model distribution. Equipmentdiscovery can occur on multiple levels of BMS 500 across multipledifferent communications busses (e.g., a system bus 554, zone buses556-560 and 564, sensor/actuator bus 566, etc.) and across multipledifferent communications protocols. In some embodiments, equipmentdiscovery is accomplished using active node tables, which provide statusinformation for devices connected to each communications bus. Forexample, each communications bus can be monitored for new devices bymonitoring the corresponding active node table for new nodes. When a newdevice is detected, BMS 500 can begin interacting with the new device(e.g., sending control signals, using data from the device) without userinteraction.

Some devices in BMS 500 present themselves to the network usingequipment models. An equipment model defines equipment objectattributes, view definitions, schedules, trends, and the associatedBACnet value objects (e.g., analog value, binary value, multistatevalue, etc.) that are used for integration with other systems. Somedevices in BMS 500 store their own equipment models. Other devices inBMS 500 have equipment models stored externally (e.g., within otherdevices). For example, a zone coordinator 508 can store the equipmentmodel for a bypass damper 528. In some embodiments, zone coordinator 508automatically creates the equipment model for bypass damper 528 or otherdevices on zone bus 558. Other zone coordinators can also createequipment models for devices connected to their zone busses. Theequipment model for a device can be created automatically based on thetypes of data points exposed by the device on the zone bus, device type,and/or other device attributes. Several examples of automatic equipmentdiscovery and equipment model distribution are discussed in greaterdetail below.

Still referring to FIG. 5, BMS 500 is shown to include a system manager502; several zone coordinators 506, 508, 510 and 518; and several zonecontrollers 524, 530, 532, 536, 548, and 550. System manager 502 canmonitor data points in BMS 500 and report monitored variables to variousmonitoring and/or control applications. System manager 502 cancommunicate with client devices 504 (e.g., user devices, desktopcomputers, laptop computers, mobile devices, etc.) via a datacommunications link 574 (e.g., BACnet IP, Ethernet, wired or wirelesscommunications, etc.). System manager 502 can provide a user interfaceto client devices 504 via data communications link 574. The userinterface may allow users to monitor and/or control BMS 500 via clientdevices 504.

In some embodiments, system manager 502 is connected with zonecoordinators 506-510 and 518 via a system bus 554. System manager 502can be configured to communicate with zone coordinators 506-510 and 518via system bus 554 using a master-slave token passing (MSTP) protocol orany other communications protocol. System bus 554 can also connectsystem manager 502 with other devices such as a constant volume (CV)rooftop unit (RTU) 512, an input/output module (TOM) 514, a thermostatcontroller 516 (e.g., a TEC5000 series thermostat controller), and anetwork automation engine (NAE) or third-party controller 520. RTU 512can be configured to communicate directly with system manager 502 andcan be connected directly to system bus 554. Other RTUs can communicatewith system manager 502 via an intermediate device. For example, a wiredinput 562 can connect a third-party RTU 542 to thermostat controller516, which connects to system bus 554.

System manager 502 can provide a user interface for any devicecontaining an equipment model. Devices such as zone coordinators 506-510and 518 and thermostat controller 516 can provide their equipment modelsto system manager 502 via system bus 554. In some embodiments, systemmanager 502 automatically creates equipment models for connected devicesthat do not contain an equipment model (e.g., IOM 514, third partycontroller 520, etc.). For example, system manager 502 can create anequipment model for any device that responds to a device tree request.The equipment models created by system manager 502 can be stored withinsystem manager 502. System manager 502 can then provide a user interfacefor devices that do not contain their own equipment models using theequipment models created by system manager 502. In some embodiments,system manager 502 stores a view definition for each type of equipmentconnected via system bus 554 and uses the stored view definition togenerate a user interface for the equipment.

Each zone coordinator 506-510 and 518 can be connected with one or moreof zone controllers 524, 530-532, 536, and 548-550 via zone buses 556,558, 560, and 564. Zone coordinators 506-510 and 518 can communicatewith zone controllers 524, 530-532, 536, and 548-550 via zone busses556-560 and 564 using a MSTP protocol or any other communicationsprotocol. Zone busses 556-560 and 564 can also connect zone coordinators506-510 and 518 with other types of devices such as variable air volume(VAV) RTUs 522 and 540, changeover bypass (COBP) RTUs 526 and 552,bypass dampers 528 and 546, and PEAK controllers 534 and 544.

Zone coordinators 506-510 and 518 can be configured to monitor andcommand various zoning systems. In some embodiments, each zonecoordinator 506-510 and 518 monitors and commands a separate zoningsystem and is connected to the zoning system via a separate zone bus.For example, zone coordinator 506 can be connected to VAV RTU 522 andzone controller 524 via zone bus 556. Zone coordinator 508 can beconnected to COBP RTU 526, bypass damper 528, COBP zone controller 530,and VAV zone controller 532 via zone bus 558. Zone coordinator 510 canbe connected to PEAK controller 534 and VAV zone controller 536 via zonebus 560. Zone coordinator 518 can be connected to PEAK controller 544,bypass damper 546, COBP zone controller 548, and VAV zone controller 550via zone bus 564.

A single model of zone coordinator 506-510 and 518 can be configured tohandle multiple different types of zoning systems (e.g., a VAV zoningsystem, a COBP zoning system, etc.). Each zoning system can include aRTU, one or more zone controllers, and/or a bypass damper. For example,zone coordinators 506 and 510 are shown as Verasys VAV engines (VVEs)connected to VAV RTUs 522 and 540, respectively. Zone coordinator 506 isconnected directly to VAV RTU 522 via zone bus 556, whereas zonecoordinator 510 is connected to a third-party VAV RTU 540 via a wiredinput 568 provided to PEAK controller 534. Zone coordinators 508 and 518are shown as Verasys COBP engines (VCEs) connected to COBP RTUs 526 and552, respectively. Zone coordinator 508 is connected directly to COBPRTU 526 via zone bus 558, whereas zone coordinator 518 is connected to athird-party COBP RTU 552 via a wired input 570 provided to PEAKcontroller 544.

Zone controllers 524, 530-532, 536, and 548-550 can communicate withindividual BMS devices (e.g., sensors, actuators, etc.) viasensor/actuator (SA) busses. For example, VAV zone controller 536 isshown connected to networked sensors 538 via SA bus 566. Zone controller536 can communicate with networked sensors 538 using a MSTP protocol orany other communications protocol. Although only one SA bus 566 is shownin FIG. 5, it should be understood that each zone controller 524,530-532, 536, and 548-550 can be connected to a different SA bus. EachSA bus can connect a zone controller with various sensors (e.g.,temperature sensors, humidity sensors, pressure sensors, light sensors,occupancy sensors, etc.), actuators (e.g., damper actuators, valveactuators, etc.) and/or other types of controllable equipment (e.g.,chillers, heaters, fans, pumps, etc.).

Each zone controller 524, 530-532, 536, and 548-550 can be configured tomonitor and control a different building zone. Zone controllers 524,530-532, 536, and 548-550 can use the inputs and outputs provided viatheir SA busses to monitor and control various building zones. Forexample, a zone controller 536 can use a temperature input received fromnetworked sensors 538 via SA bus 566 (e.g., a measured temperature of abuilding zone) as feedback in a temperature control algorithm. Zonecontrollers 524, 530-532, 536, and 548-550 can use various types ofcontrol algorithms (e.g., state-based algorithms, extremum seekingcontrol (ESC) algorithms, proportional-integral (PI) control algorithms,proportional-integral-derivative (PID) control algorithms, modelpredictive control (MPC) algorithms, feedback control algorithms, etc.)to control a variable state or condition (e.g., temperature, humidity,airflow, lighting, etc.) in or around building 10.

HVAC System with Free Cooling

Referring now to FIG. 6, a block diagram of a HVAC system 600 with freecooling is shown, according to an exemplary embodiment. HVAC system 600is configured to provide cooling to a cooling load 608. Cooling load 608can include, for example, a building zone, a supply airstream flowingthrough an air duct, an airflow in an air handling unit or rooftop unit,fluid flowing through a heat exchanger, a refrigerator or freezer, acondenser or evaporator, a cooling coil, or any other type of system,device, or space which requires cooling. In some embodiments, a pump 622circulates a chilled fluid to cooling load 608 via a chilled fluidcircuit 636. The chilled fluid can absorb heat from cooling load 608,thereby providing cooling to cooling load 608 and warming the chilledfluid.

HVAC system 600 is shown to include a cooling tower 602, a heatexchanger 606, and a chiller 610. HVAC system 600 can operate in both amechanical cooling state (shown in FIG. 7) and a free cooling state(shown in FIG. 8). HVAC system 600 can transition between the mechanicalcooling state and free cooling state to provide economically optimalcooling for cooling load 608. In the mechanical cooling state, thechilled fluid exiting cooling load 608 is directed to an evaporator 616of chiller 610. Chiller 610 operates to provide mechanical cooling(e.g., vapor compression cooling) for the chilled fluid in evaporator616 by transferring heat from the chilled fluid to a refrigerant whichcirculates through evaporator 616 via a refrigeration circuit 634. Inthe free cooling state, the chilled fluid exiting cooling load 608 isdirected to a heat exchanger 606. Heat exchanger 606 is configured totransfer heat from the chilled fluid to water (or any other coolant)which circulates through heat exchanger 606 via a cooling tower circuit632.

Cooling tower 602 can be configured to cool the water in cooling towercircuit 632 by transferring heat from the water to outside air. In someembodiments, a pump 620 circulates water through cooling tower 602 viacooling tower circuit 632. Cooling tower 602 may include a fan 604 whichcauses cool air to flow through cooling tower 602. Cooling tower 602places the cool air in a heat exchange relationship with the warmerwater, thereby transferring heat from warmer water to the cooler air. Inthe mechanical cooling state, cooling tower 602 can provide cooling fora condenser 612 of chiller 610. Condenser 612 can transfer heat from therefrigerant in refrigeration circuit 634 to the water in cooling towercircuit 632. In the free cooling state, cooling tower 602 can providecooling for heat exchanger 606. Heat exchanger 606 can transfer heatfrom the chilled fluid in chilled fluid circuit 636 to the water incooling tower circuit 632. Although cooling tower circuit 632 is shownand described as circulating water, it should be understood that anytype of coolant or working fluid (e.g., water, glycol, CO2, etc.) can beused in cooling tower circuit 632.

Chiller 610 is shown to include a condenser 612, a compressor 614, anevaporator 616, and an expansion device 618. Compressor 614 can beconfigured to circulate a refrigerant between condenser 612 andevaporator 616 via refrigeration circuit 634. Compressor 614 operates tocompress the refrigerant to a high pressure, high temperature state. Thecompressed refrigerant flows through condenser 612, which transfers heatfrom the refrigerant in refrigeration circuit 634 to the water incooling tower circuit 632. The cooled refrigerant then flows throughexpansion device 618, which expands the refrigerant to a lowtemperature, low pressure state. The expanded refrigerant flows throughevaporator 616, which transfers heat from the chilled fluid in chilledfluid circuit 636 to the refrigerant in refrigeration circuit 634.

In some embodiments, chiller 610 is active only when HVAC systemoperates in the mechanical cooling state. In the free cooling state,chiller 610 can be deactivated to reduce energy consumption. In someembodiments, HVAC system 600 includes multiple chillers 610. Each ofchillers 610 can be arranged in parallel and configured to providecooling for the fluid in chilled fluid circuit 636. Similarly, HVACsystem 600 can include multiple cooling towers 602. Each of the coolingtowers 602 can be arranged in parallel and configured to provide coolingfor the water in cooling tower circuit 632.

Still referring to FIG. 6, HVAC system 600 is shown to include severalvalves 624, 626, 628, and 630. Valves 624-630 may be three-way valveswhich can be operated by a controller 640 to control the flow of thechilled fluid in chilled fluid circuit 636 and the water in coolingtower circuit 632. For example, when HVAC system 600 transitions intothe mechanical cooling state, controller 640 can operate valves 628 and630 to direct the chilled fluid exiting cooling load 608 throughevaporator 616 and prevent the chilled fluid from flowing through heatexchanger 606. In the mechanical cooling state, controller 640 canoperate valves 624 and 626 to direct the water exiting cooling tower 602through condenser 612 and prevent the water from flowing through heatexchanger 606. Conversely, when HVAC system 600 transitions into thefree cooling state, controller 640 can operate valves 628 and 630 todirect the chilled fluid exiting cooling load 608 through heat exchanger606 and prevent the chilled fluid from flowing through evaporator 616.In the free cooling state, controller 640 can operate valves 624 and 626to direct the water exiting cooling tower 602 through heat exchanger 606and prevent the water from flowing through condenser 612.

Referring now to FIG. 7, a block diagram illustrating the operation ofHVAC system 600 in the mechanical cooling state is shown, according toan exemplary embodiment. In FIG. 7, the flow paths used in themechanical cooling state are shown in solid lines, whereas the flowpaths not used in the mechanical cooling state are shown in brokenlines. In the mechanical cooling state, chiller 610 is used to providecooling for the chilled fluid in chilled fluid circuit 636. Both chilledfluid circuit 636 and cooling tower circuit 632 are fluidly connected tochiller 610. Heat exchanger 606 is not used and the fluid conduitsconnecting to heat exchanger 606 are blocked.

In the mechanical cooling state, controller 640 operates valve 624 todirect the cool water from cooling tower 602 through condenser 612.Condenser 612 transfers heat from the refrigerant in refrigerationcircuit 634 to the cool water in cooling tower circuit 632, therebywarming the water. The warm water then flows from condenser 612 to valve626. Controller 640 operates valve 626 to direct the warm water tocooling tower 602. Cooling tower 602 transfers heat from the water tocooler air flowing through cooling tower 602. Controller 640 can operatefan 604 to modulate the airflow through cooling tower 602, which adjuststhe rate of heat transfer in cooling tower 602. Controller 640 can alsooperate pump 620 to modulate the flow rate of the water through coolingtower circuit 632, which adjusts the rate of heat transfer in coolingtower 602 and/or condenser 612.

In the mechanical cooling state, controller 640 operates valve 630 todirect the fluid exiting cooling load 608 through evaporator 616.Evaporator 616 transfers heat from the fluid in chilled fluid circuit636 to the refrigerant in refrigeration circuit 634, thereby chillingthe fluid in chilled fluid circuit 636. The chilled fluid then flowsfrom evaporator 616 to valve 628. Controller 640 operates valve 628 todirect the chilled fluid to cooling load 608. Cooling load 608 rejectsheat to the chilled fluid, thereby providing cooling for cooling load608 and warming the chilled fluid. Controller 640 can operate pump 622to modulate the flowrate of the chilled fluid through chilled fluidcircuit 636, which adjusts the rate of heat transfer in evaporator 616and/or at cooling load 608.

Referring now to FIG. 8, a block diagram illustrating the operation ofHVAC system 600 in the free cooling state is shown, according to anexemplary embodiment. In FIG. 8, the flow paths used in the free coolingstate are shown in solid lines, whereas the flow paths not used in thefree cooling state are shown in broken lines. In the free cooling state,heat exchanger 606 is used to provide cooling for the chilled fluid inchilled fluid circuit 636. Both chilled fluid circuit 636 and coolingtower circuit 632 are fluidly connected to heat exchanger 606. Chiller610 is not used and the fluid conduits connecting to chiller 610 areblocked.

In the free cooling state, controller 640 operates valve 624 to directthe cool water from cooling tower 602 through heat exchanger 606. Heatexchanger 606 transfers heat from the fluid in chilled fluid circuit tothe cool water in cooling tower circuit 632, thereby warming the water.The warm water then flows from heat exchanger 606 to valve 626.Controller 640 operates valve 626 to direct the warm water to coolingtower 602. Cooling tower 602 transfers heat from the water to cooler airflowing through cooling tower 602. Controller 640 can operate fan 604 toincrease or decrease the airflow through cooling tower 602, whichincreases or decreases the rate of heat transfer in cooling tower 602.Controller 640 can also operate pump 620 to modulate the flow rate ofthe water through cooling tower circuit 632, which adjusts the rate ofheat transfer in cooling tower 602 and/or heat exchanger 606.

In the free cooling state, controller 640 operates valve 630 to directthe fluid exiting cooling load 608 through heat exchanger 606. Heatexchanger 606 transfers heat from the fluid in chilled fluid circuit 636to the water in cooling tower circuit 632, thereby chilling the fluid inchilled fluid circuit 636. The chilled fluid then flows from heatexchanger 606 to valve 628. Controller 640 operates valve 628 to directthe chilled fluid to cooling load 608. Cooling load 608 rejects heat tothe chilled fluid, thereby providing cooling for cooling load 608 andwarming the chilled fluid. Controller 640 can operate pump 622 tomodulate the flowrate of the chilled fluid through chilled fluid circuit636, which adjusts the rate of heat transfer in heat exchanger 606and/or at cooling load 608.

HVAC Controller

Referring now to FIG. 9, a block diagram illustrating a portion of HVACsystem 600 and controller 640 in greater detail is shown, according toan exemplary embodiment. In brief overview, controller 640 receivesmeasurements from sensors 914 and weather forecasts from a weatherservice 916. Controller 640 uses the sensor measurements and weatherforecasts to determine an operating state for HVAC system 600. Forexample, controller 640 can determine whether to transition into amechanical cooling state, a free cooling state, or a no cooling state.Controller 640 can generate and provide control signals for HVACequipment 930 (e.g., valves 624-630, chiller 610, etc.). HVAC equipment930 operate to affect an environmental condition in a building (e.g.,temperature, humidity, airflow, etc.), which can be measured by sensors914 and provided as a feedback to controller 640.

Controller 640 can be any type of controller in a HVAC system or BMS. Insome embodiments, controller 640 is a zone controller configured tomonitor and control a building zone. For example, controller 640 can bea zone temperature controller, a zone humidity controller, a zonelighting controller, a VAV zone controller (e.g., VAV zone controllers524, 532, 536, 550), a COBP zone controller (e.g., COPB controller 530,548), or any other type of controller for a building zone. In otherembodiments, controller 640 is a system controller or subsystemcontroller. For example, controller 640 can be a BMS controller (e.g.,BMS controller 366), a central plant controller, a subplant controller,a supervisory controller for a HVAC system or any other type of buildingsubsystem (e.g., a controller for any of building subsystems 428). Insome embodiments, controller 640 is a field controller or devicecontroller configured to monitor and control the performance of a set ofHVAC devices or other building equipment. For example, controller 640can be an AHU controller (e.g., AHU controller 330), a thermostatcontroller (e.g., thermostat controller 516), a rooftop unit controller,a chiller controller, a damper controller, or any other type ofcontroller in a HVAC system or BMS.

In some embodiments, controller 640 is a hybrid controller whichcombines the functionality of a discrete control system and a closedloop control system. A discrete control system can be described using afinite state diagram (FSD) and implemented in a finite state machine(FSM). In a discrete control system, a controller evaluates statetransition conditions (e.g., using feedback from the controlled system)and transitions between various operating states when one or more of thestate transition conditions are satisfied. Each of the operating statesin a discrete control system can have a corresponding set of controloutputs. In some embodiments, the control outputs in a discrete controlsystem remain constant as long as the controller remains in the sameoperating state and change only when the controller transitions into anew operating state.

A closed loop control system can be implemented using any of a varietyof control techniques (e.g., feedback control, feedforward control,extremum seeking control, proportional-integral control,proportional-integral-derivative control, model predictive control,etc.). In a closed loop control system, a controller modulates a controloutput (i.e., a manipulated variable) provided to the controlled systemover a range of values in order to achieve a desired effect. Forexample, the controller can modulate the control output to drive amonitored variable to a setpoint. In some embodiments, the controlleruses feedback from the controlled system to determine an error betweenthe setpoint and the monitored variable. The controller can variablyincrease or decrease the control output within the range of values inorder to drive the error to zero.

Controller 640 can include both discrete control elements and closedloop control elements. For example, controller 640 is shown to include astate transition controller 910 and a plurality of state controllers 920(i.e., mechanical cooling state controller 922, free cooling statecontroller 924, and no cooling state controller 926). State transitioncontroller 910 can operate as a finite state machine to evaluate statetransition conditions and transition between various operating states.The state transition conditions and the logic used by state transitioncontroller 910 can be stored in a database for later retrieval. In someembodiments, state transition controller 910 provides an indication ofthe current operating state to state controllers 920. State transitioncontroller 910 is described in greater detail below.

Each of state controllers 920 can operate as a closed loop controllerwithin a particular operating state. In some embodiments, each statecontroller 920 becomes active when state transition controller 910transitions into the corresponding operating state and inactive whenstate transition controller 910 transitions out of the correspondingoperating state. In some embodiments, each of state controllers 920 usesa different control algorithm and/or different control logic. Thisallows controller 640 to function as multiple different controllers,each of which controls the operation of system 600 in a particularoperating state. State controllers 920 are described in greater detailbelow.

Still referring to FIG. 9, HVAC system 600 is shown to include sensors914, weather service 916, user devices 918, and HVAC equipment 930.Sensors 914 can include any of a variety of sensors configured tomeasure a variable state or condition in a building. For example,sensors 914 can include temperature sensors, humidity sensors, airflowsensors, lighting sensors, pressure sensors, voltage sensors, or anyother type of sensor. Sensors 914 can be distributed throughout abuilding and configured to measure various environmental conditions atdifferent locations in the building. For example, one of sensors 914 canbe located in a first zone of the building and configured to measure thetemperature of the first zone, whereas another of sensors 914 can belocated in a second zone of the building and configured to measure thetemperature of the second zone. Similarly, sensors 914 can bedistributed throughout a HVAC system and configured to measureconditions at different locations in the HVAC system. For example, oneof sensors 914 can be a supply air temperature sensor configured tomeasure the temperature of the airflow provided to a building zone froman AHU, whereas another of sensors 914 can be a return air temperaturesensor configured to measure the temperature of the airflow returningfrom the building zone to the AHU.

In some embodiments, sensors 914 include outdoor air sensors configuredto measure the temperature, pressure, humidity, or other attributes ofthe air outside the building. Sensors 914 can provide measurements asinputs to controller 640 via communications interface 902. In someembodiments, sensors 914 provide a feedback signal to controller 640indicating the value of a variable of interest in the controlled system(e.g., building zone temperature, building zone humidity, system powerconsumption, etc.) or outside the controlled system (e.g., outdoor wetbulb air temperature). Controller 640 can use the measurements fromsensors 914 to evaluate state transition conditions and/or to performclosed loop control operations within various operating states.

Weather service 916 can be configured to provide weather forecasts tocontroller 640. The weather forecasts can include temperature forecasts,humidity forecasts, wind forecasts, rain or snow forecasts, or any othertype of weather forecast. Controller 640 can use the weather forecaststo predict the temperature, humidity, wet bulb temperature, or otherattributes of the outdoor air at a plurality of future times. In someembodiments, controller 640 uses the predicted attributes of the outdoorair to evaluate state transition conditions and/or to perform closedloop control operations within various operating states. The logic usedby controller 640 to evaluate state transition conditions and performstate transitions is described in greater detail below.

User devices 918 can include any of a variety of user-operable devicesconfigured to facilitate user interaction with controller 640 and/orHVAC system. For example, user devices 918 can include a computerworkstation, a desktop computer, a laptop computer, a tablet, asmartphone, a PDA, or any other type of mobile or non-mobile device.User devices 918 can include user interface elements (e.g., electronicdisplay screens, touchscreen displays, keyboards, speakers, buttons,dials, etc.) configured to receive input from a user and provide outputto a user. User devices 918 can interact with controller 640 viacommunications interface 912 to monitor system operation and provideinput to controller 640. For example, user devices 918 can allow a userto provide controller 640 with setpoints, operating parameters, manualvalues for measured variables, operating commands, manual statetransition commands, and/or other types of user input. Controller 640can use the input from user devices 918 to evaluate state transitionconditions and/or to perform closed loop control operations withinvarious operating states.

HVAC equipment 930 can include any of a variety of controllable systemsor devices in HVAC system 600. For example, HVAC equipment 930 caninclude cooling tower 602, fan 604, chiller 610, pumps 620-622, and/orvalves 624-630. HVAC equipment 930 can include any of the systems ordevices of HVAC system 100, waterside system 200, or airside system 300,as described with reference to FIGS. 1-3. For example, HVAC equipment930 can include one or more chillers, boilers, AHUs, economizers,controllers, actuators, fans, pumps, electronic valves, and/or othertypes of equipment which can be operated by controller 640 to affect avariable state or condition (e.g., temperature, humidity, airflow,lighting, etc.) in or around building 10.

HVAC equipment 930 can include any of the systems or devices of buildingsubsystems 428 as described with reference to FIG. 4 and/or any of thesystems or devices of BMS 500 as described with reference to FIG. 5(e.g., zone coordinators, rooftop units, VAV units, bypass dampers,etc.). HVAC equipment 930 can provide operating data to controller 640and can receive control signals from controller 640. In someembodiments, HVAC equipment 930 operate according to the control signalsto affect one or more of the variables measured by sensors 914.

Still referring to FIG. 9, controller 640 is shown to include acommunications interface 912 and a processing circuit 904.Communications interface 912 can include wired or wireless interfaces(e.g., jacks, antennas, transmitters, receivers, transceivers, wireterminals, etc.) for conducting data communications with varioussystems, devices, or networks. For example, communications interface 912can include an Ethernet card and port for sending and receiving data viaan Ethernet-based communications network and/or a WiFi transceiver forcommunicating via a wireless communications network. Communicationsinterface 912 can be configured to communicate via local area networksor wide area networks (e.g., the Internet, a building WAN, etc.) and mayuse a variety of communications protocols (e.g., BACnet, IP, LON, etc.).

Communications interface 912 can be a network interface configured tofacilitate electronic data communications between controller 640 andvarious external systems or devices (e.g., sensors 914, weather service916, user devices 918, HVAC equipment 930, etc.). For example,controller 640 can receive setpoints and operating parameters from asupervisory controller (e.g., BMS controller 366, system manager 502,etc.) via communications interface 912. Controller 640 can receivemeasurements from sensors 914 via communications interface 912.Controller 640 can use communications interface 912 to send controlsignals to HVAC equipment 930. In some embodiments, controller 640provides user interfaces and other information to user devices 918 viacommunications interface 912.

Processing circuit 904 is shown to include a processor 906 and memory908. Processor 906 can be a general purpose or specific purposeprocessor, an application specific integrated circuit (ASIC), one ormore field programmable gate arrays (FPGAs), a group of processingcomponents, or other suitable processing components. Processor 906 canbe configured to execute computer code or instructions stored in memory908 or received from other computer readable media (e.g., CDROM, networkstorage, a remote server, etc.).

Memory 908 can include one or more devices (e.g., memory units, memorydevices, storage devices, etc.) for storing data and/or computer codefor completing and/or facilitating the various processes described inthe present disclosure. Memory 908 can include random access memory(RAM), read-only memory (ROM), hard drive storage, temporary storage,non-volatile memory, flash memory, optical memory, or any other suitablememory for storing software objects and/or computer instructions. Memory908 can include database components, object code components, scriptcomponents, or any other type of information structure for supportingthe various activities and information structures described in thepresent disclosure. Memory 908 can be communicably connected toprocessor 906 via processing circuit 904 and can include computer codefor executing (e.g., by processor 906) one or more processes describedherein.

State Transitions and Operating States

Referring now to FIG. 10, a state transition diagram 1000 illustratingthe operation of HVAC system 600 is shown, according to an exemplaryembodiment. State transition diagram 1000 is shown to include aplurality of operating states 1002-1006 (i.e., a mechanical coolingstate 1002, a free cooling state 1004, and a no cooling state 1006) andstate transition conditions 1008-1014. Although only three operatingstates 1002-1006 are shown in state transition diagram 1000, it shouldbe understood that state transition diagram 1000 can include any numberof operating states to model systems of various complexity. In someembodiments, various sub-states can be nested within one or more ofoperating states 1002-1006. However, such sub-states are omitted fromstate transition diagram 1000 for simplicity.

State transition controller 910 can evaluate state transition conditions1008-1014 and can transition between operating states 1002-1006 based ona result of the evaluation. State transition conditions 1008-1014 caninvolve time comparisons and/or value comparisons. In traditional freecooling systems, free cooling is typically used whenever the outdoor wetbulb air temperature is below a minimum temperature required for freecooling. However, the traditional approach does not take into accountthe economic cost associated with transitioning between operatingstates. For example, switching between mechanical cooling state 1002 andfree cooling state 1004 may incur an economic cost. The economic costmay result from increased electricity consumption when chiller 610 isstarting-up, increased equipment degradation resulting from switchingchiller 610 on/off, inefficient chiller operation while chiller 610 isstarting-up, electricity required to operate valves 624-630, and/or anyother economic costs which are incurred as a result of the statetransition.

To make free cooling economically viable, the energy and cost savingsachieved by free cooling should be sufficient to overcome the costincurred as a result of transitioning between mechanical cooling state1002 and free cooling state 1004. Advantageously, state transitioncontroller 910 can determine whether the use of free cooling would beeconomically viable by weighing the cost savings achieved by freecooling against the economic cost of performing the state transition.For example, free cooling may be economically viable only if the freecooling lasts for a minimum amount of time. State transition controller910 can predict how long the use of free cooling would last as well asthe energy savings which would be achieved by the use of free coolingduring the predicted free cooling period. State transition controller910 can weigh the predicted energy savings against the cost ofperforming the state transition to determine whether to transition intofree cooling state 1004.

In some embodiments, state transition controller 910 is configured topredict the outside air temperature {circumflex over (T)}_(OA) (e.g.,predicted outside air wet bulb temperature) for each of a plurality oftime steps into the future. State transition controller 910 can predictthe outside air temperature {circumflex over (T)}_(OA) usingmeasurements from sensors 914 and/or weather forecasts from weatherservice 916. When operating in mechanical cooling state 1002, statetransition controller 910 can determine whether the predicted outsideair temperature {circumflex over (T)}_(OA) will be below a free coolingtemperature threshold T_(FC) for a predetermined amount of time in thefuture (transition condition 1008). State transition controller 910 cantransition from mechanical cooling state 1002 to free cooling state 1004in response to a determination that state transition condition 1008 issatisfied.

In some embodiments, the free cooling temperature threshold T_(FC) is amaximum outdoor air wet bulb temperature at which free cooling ispossible or economically viable. The predetermined amount of time may bea minimum amount of time t_(min,FC) which free cooling must last inorder to justify the economic cost of transitioning into free coolingstate 1004. If the predicted outside air temperature {circumflex over(T)}_(OA) will not stay below the temperature threshold T_(FC) for thepredetermined amount of time t_(min,FC,) state transition controller 910can remain in mechanical cooling state 1002, even if the current outsideair temperature T_(OA) is below the temperature threshold T_(FC). Thisprevents state transition controller 910 from transitioning into freecooling state 1004 if the amount of time spent in free cooling state1004 and the corresponding energy savings are insufficient to overcomethe cost incurred as a result of the state transition.

In some embodiments, state transition controller 910 calculates theminimum free cooling time t_(min,FC.) State transition controller 910can calculate the minimum free cooling time t_(min,FC) by weighing thefree cooling energy savings against the cost incurred as a result ofswitching from mechanical cooling state 1002 to free cooling state 1004.For example, state transition controller 910 can use the followingequation to calculate the economic value of transitioning into freecooling state 1004 and operating in free cooling state 1004:

Value_(FC) =Δt _(FC)Cost_(elec) P _(elec)−SwitchingPenalty

where Value_(FC) is the total economic value of transitioning into freecooling state 1004 and operating in free cooling state 1004 during thepredicted free cooling period, Δt_(FC) is the duration of the freecooling period (i.e., the predicted amount of time which will be spentin free cooling state 1004), Cost_(elec) is the estimated per unit costof electricity during the free cooling period

$( {{e.g.},\frac{\$}{kWh}} ),$

P_(elec) is the estimated free cooling energy savings per unit timeduring the free cooling period (e.g., kW), and Switching Penalty is theeconomic or monetary cost (e.g., $) incurred as a result of switchingfrom mechanical cooling state 1002 to free cooling state 1004.

In the previous equation, the term Δt_(FC)Cost_(elec)P_(elec) representsthe cost savings resulting from the use of free cooling relative tomechanical cooling over the duration of the free cooling period. Forexample, the product of energy cost Cost_(elec)

$( {{e.g.},\frac{\$}{kWh}} )$

and energy savings per unit time P_(elec) (e.g., kW) represents theeconomic cost of electricity which is saved by the use of free coolingduring each time step of the free cooling period

$( {{e.g.},\frac{\$}{hour}} ).$

Multiplying this savings per unit time by the duration of the freecooling period Δt_(FC) (e.g., hours) results in the total cost savingsover the duration of the free cooling period. The term SwitchingPenaltyrepresents the economic cost incurred as a result of the statetransition. As previously described, the economic cost may result fromincreased electricity consumption during chiller start-up or shut-down,increased equipment degradation resulting from switching chiller 610on/off, inefficient chiller operation while chiller 610 is starting-upor shutting-down, electricity required to operate valves 624-630, and/orany other economic costs which are incurred as a result of the statetransition.

State transition controller 910 can calculate the minimum free coolingtime by finding the duration of the free cooling period Δt_(FC) whichresults in a total economic value of zero (i.e., Value=0). For example,state transition controller 910 can solve the following equation tocalculate the minimum free cooling time t_(min,FC):

0 = t_(min , FC)Cost_(elec)P_(elec) − SwitchingPenalty$t_{\min,{FC}} = \frac{{Switching}\; {Penalty}}{{Cost}_{elec}P_{elec}}$

where the values of Cost_(elec), P_(elec), and SwitchingPenalty haveknown values. The value of SwitchingPenalty can be fixed, whereas thevalue of Cost_(elec) can be received from an energy utility or predictedbased upon past costs of electricity. The value of P_(elec) can bepredicted or estimated based on the amount of cooling required bycooling load 608.

When operating in free cooling state 1004, state transition controller910 can determine whether the predicted outside air temperature{circumflex over (T)}_(OA) will be above the temperature thresholdT_(FC) for a predetermined amount of time in the future (transitioncondition 1010). State transition controller 910 can transition fromfree cooling state 1004 to mechanical cooling state 1002 in response toa determination that state transition condition 1010 is satisfied. Thepredetermined amount of time in state transition condition 1010 can be aminimum mechanical cooling time t_(min,FC) required to justifytransitioning into mechanical cooling state 1002. The minimum mechanicalcooling time t_(min,FC) in state transition condition 1010 can be thesame or different from the minimum free cooling time t_(min,FC) in statetransition condition 1008.

When operating in free cooling state 1004, state transition controller910 can determine whether the actual outside air temperature T_(OA) isabove the temperature threshold T_(FC) (transition condition 1012).State transition controller 910 can transition from free cooling state1004 to no cooling state 1006 in response to a determination that statetransition condition 1012 is satisfied. In no cooling state 1006,neither free cooling nor mechanical cooling are used. A transition intono cooling state 1006 may occur when the outside air temperature T_(OA)is above the free cooling temperature threshold T_(FC), but is notpredicted to remain above the temperature threshold T_(FC) for theminimum amount of time t_(min,FC) required to justify switching back tomechanical cooling. State transition controller 910 may remain in nocooling state 1006 until the actual outside air temperature T_(OA) dropsbelow the temperature threshold T_(FC).

When operating in no cooling state 1006, state transition controller 910can determine whether the actual outside air temperature T_(OA) is belowthe temperature threshold T_(FC) (transition condition 1014). Statetransition controller 910 can transition from no cooling state 1006 tofree cooling state 1004 in response to a determination that statetransition condition 1012 is satisfied. In some embodiments, statetransition controller 910 transitions from no cooling state 1006 tomechanical cooling state 1002 in response to a determination that thepredicted outside air temperature {circumflex over (T)}_(OA) will beabove the temperature threshold T_(FC) for an amount of time exceedingthe minimum mechanical cooling time t_(min,FC). However, such a statetransition may not be necessary because state transition controller 910may not operate in no cooling state 1006 unless the outside airtemperature T_(OA) is predicted to drop below the temperature thresholdT_(FC) within the minimum mechanical cooling time t_(min,FC).

In some embodiments, state transition controller 910 determines whetherto use free cooling or mechanical cooling at each of a plurality of timesteps k within a horizon of duration h by optimizing a cost function Jover the horizon. At each time step k, the cost function J can bewritten as follows:

J _(k) =C*

where J_(k) is the value of the cost function at time step k, C is acost vector, and

is a vector of decision variables at time step k. The vector of decisionvariables

may include binary decision variables and/or continuous decisionvariables that indicate whether free cooling or mechanical cooling willbe used during time step k (described in greater detail below). The costvector C can include cost parameters that indicate an economic costassociated with each of the decision variables.

The total cost over the horizon can be expressed as follows:

$J_{total} = {{\sum\limits_{k = 1}^{h}J_{k}} = {\sum\limits_{k = 1}^{h}{C \star \overset{arrow}{x_{k}}}}}$

State transition controller 910 can determine optimal values for thedecision variables in vector

at each time step k by optimizing (i.e., minimizing) the total costJ_(total) over the horizon. Accordingly, the optimization problem can beformulated as shown in the following equation:

${\min \mspace{11mu} ( J_{total} )} = {{\min \mspace{11mu} ( {\sum\limits_{k = 1}^{h}J_{k}} )} = {\min \mspace{11mu} ( {\sum\limits_{k = 1}^{h}{C \star \overset{arrow}{x_{k}}}} )}}$

In other embodiments, the decision vector

can be replaced with a decision matrix X. Each column of the decisionmatrix X may be the decision vector

for a particular time step k and may include the values of the decisionvariables for that time step. Each row of the decision matrix X maycorrespond to a particular decision variable. Each element of thedecision matrix X (i.e., the intersection of a row and column) mayindicate the value of the corresponding decision variable during thecorresponding time step. With the decision matrix X, the total cost overthe horizon can be expressed as follows:

J _(total) =C*X

and optimization problem can be formulated as shown in the followingequation:

min(J _(total))=min(C*X)

In some embodiments, state transition controller 910 uses a binarypenalty approach to define the decision vector

and the cost vector C. When the binary penalty approach is used, thedecision vector

can be defined as follows:

=[X _(1,k) X _(2,k) X _(3,k) ,b _(1,k) ,b _(2,k) ,P _(k)]^(T)

where X_(1,k) is the cooling load allocated to free cooling (e.g., to afree cooling subplant) at time step k, X_(2,k) is the cooling loadallocated to mechanical cooling (e.g., to a mechanical cooling subplant)at time step k, and X_(3,k) is the remaining cooling load at time stepk. The remaining cooling load X_(3,k) can be designated as an unmetcooling load or allocated to another plant or subplant (e.g., thermalenergy storage). The variable b_(1,k) is a binary decision variablewhich indicates whether free cooling will be used during time step k.Similarly, the variable b_(2,k) is a binary decision variable whichindicates whether mechanical cooling will be used during time step k.The variable P_(k) indicates whether the switching penalty is activeduring time step k.

When the binary penalty approach is used, the cost vector C can bedefined as follows:

C=[λ₁ C _(u),λ₂ C _(u),λ₃ C _(u),0,0,C _(p)]

where λ₁ is the efficiency of the free cooling subplant, λ₂ is theefficiency of the mechanical cooling subplant, λ₃ is the efficiency ofthe subplant which is allocated cooling load X_(3,k), C_(u) is theenergy usage cost, and C_(p) is the switching penalty cost.

When the binary approach is used, state transition controller 910 canoptimize the total cost J_(total) subject to the following constraints:

b ₁ ,b ₂∈{0,1}

b _(1,k) +b _(2,k)=1

X _(1,k) +X _(2,k) +X _(3,k) =Q _(Load,k)

−b _(1,k) X _(1,Max) +X _(1,k)≤0

−b _(2,k) X _(2,Max) +X _(2,k)≤0

X _(1,k) ≤X _(1,Max)FCAVail_(k)

b _(1,k) −b _(1,k-1) −P _(k)≤0

b _(1,k-1) −b _(1,k) −P _(k)≤0

where Q_(Load,k) is the total cooling load to be met at time step k,X_(1,Max) is the maximum capacity of the free cooling subplant,X_(2,Max) is the maximum capacity of the mechanical cooling subplant,FCAvail_(k) is a binary variable that indicates whether free cooling isavailable at time step k (e.g., FCAvail_(k)=1) or unavailable at timestep k (e.g., FCAvail_(k)=0), and is a binary variable which indicateswhether free cooling will be used at time step k−1.

In some embodiments, state transition controller 910 sets the value forFCAvail_(k) based on the predicted outside wet bulb air temperatureT_(OA) at time step k. For example, state transition controller 910 canset FCAvail_(k)=1 if the predicted wet bulb air temperature T_(OA) attime step k is below the free cooling temperature threshold T_(FC).Similarly, state transition controller 910 can set FCAvail_(k)=0 if thepredicted wet bulb air temperature T_(OA) at time step k is above thefree cooling temperature threshold T_(FC).

In some embodiments, state transition controller 910 uses a continuouspenalty approach to define the decision vector

and the cost vector C. When the continuous penalty approach is used, thedecision vector

can be defined as follows:

=[X _(1,k) ,X _(2,k) X _(3,k)FCAvail_(k),δ_(1,k) ⁺,δ_(2,k) ⁺,δ_(3,k)⁺,δ_(1,k) ⁻,δ_(2,k) ⁻,δ_(3,k) ⁻]

where X_(1,k) is the cooling load allocated to free cooling (e.g., to afree cooling subplant) at time step k, X_(2,k) is the cooling loadallocated to mechanical cooling (e.g., to a mechanical cooling subplant)at time step k, and X_(3,k) is the remaining cooling load at time stepk. The remaining cooling load X_(3,k) can be designated as an unmetcooling load or allocated to another plant or subplant (e.g., thermalenergy storage). The variable FCAvail_(k) is a binary variable thatindicates whether free cooling is available at time step k (e.g.,FCAvail_(k)=1) or unavailable at time step k (e.g., FCAvail_(k)=), aspreviously described.

The variables δ_(1,k) ⁺, λ_(2,k) ⁺, and δ_(3,k) ⁺ indicate the amounts(if any) by which the cooling loads X_(1,k), X_(2,k), and X_(3,k)increased relative to their values at the previous time step k−1. Forexample, the variables δ_(1,k) ⁺, δ_(2,k) ⁺, and δ_(3,k) ⁺ can bedefined as follows:

$\begin{matrix}{\delta_{1,k}^{+} = {\max \{ \begin{matrix}0 \\{X_{1,k} - X_{1,{k - 1}}}\end{matrix} }} \\{\delta_{2,k}^{+} = {\max \{ \begin{matrix}0 \\{X_{2,k} - X_{2,{k - 1}}}\end{matrix} }} \\{\delta_{3,k}^{+} = {\max \{ \begin{matrix}0 \\{X_{3,k} - X_{3,{k - 1}}}\end{matrix} }}\end{matrix}$

Similarly, the variables δ_(1,k) ⁻, δ_(2,k) ⁻, and δ_(3,k) ⁻ indicatethe amounts (if any) by which the cooling loads X_(1,k), X_(2,k), andX_(3,k) decreased relative to their values at the previous time stepk−1. For example, the variables δ_(1,k) ⁻, δ_(2,k) ⁻, and δ_(3,k) ⁻ canbe defined as follows:

$\begin{matrix}{\delta_{1,k}^{-} = {\max \{ \begin{matrix}0 \\{X_{1,{k - 1}} - X_{1,k}}\end{matrix} }} \\{\delta_{2,k}^{-} = {\max \{ \begin{matrix}0 \\{X_{2,{k - 1}} - X_{2,k}}\end{matrix} }} \\{\delta_{3,k}^{-} = {\max \{ \begin{matrix}0 \\{X_{3,{k - 1}} - X_{3,k}}\end{matrix} }}\end{matrix}$

When the continuous penalty approach is used, the cost vector C can bedefined as follows:

C=[λ₁ C _(u),λ₂ C _(u),λ₃ C _(u),0,c ₁ ^(Δ) ,c ₂ ^(Δ) ,c ₃ ^(Δ) ,c ₁^(Δ) ,c ₂ ^(Δ) ,c ₃ ^(Δ)]

where λ₁ is the efficiency of the free cooling subplant, λ₂ is theefficiency of the mechanical cooling subplant, λ₃ is the efficiency ofthe subplant which is allocated cooling load X_(3,k), c₁ ^(Δ) is thepenalty cost of a load change in the free cooling subplant, c₂ ^(Δ) isthe penalty cost of a load change in the mechanical cooling subplant,and c₃ ^(Δ) is the penalty cost of a load change in the subplant whichis allocated cooling load X_(3,k).

When the continuous penalty approach is used, state transitioncontroller 910 can optimize the total cost J_(total) subject to thefollowing constraints:

FCAvail_(k)∈{0,1}

X _(n,k)≥0

δ_(n,k) ⁺≥0

δ_(n,k) ⁻≥0

X _(1,k) +X _(2,k) +X _(3,k) =Q _(Load,k)

X _(n,k) ≤X _(n,Max)

X _(1,k) ≤M _(big)FCAvail_(k)

X _(2,k) ≤M _(big)(1−FCAvail_(k))

(T _(OA,k) −T _(FC))−M _(big)(1−FCAvail_(k))≤0

where X_(n,k) is the cooling load allocated to subplant n at time step k(n=1 . . . 3), δ_(n,k) ⁺ is the increase (if any) in the cooling loadallocated to subplant n between time step k−1 and time step k, δ_(n,k) ⁻is the decrease (if any) in the cooling load allocated to subplant nbetween time step k−1 and time step k, X_(n,Max) is the maximum capacityof subplant n, Q_(Load,k) is the total cooling load to be met at timestep k, T_(OA,k) is the predicted outside air wet bulb temperature attime step k, T_(FC) is the free cooling temperature threshold (e.g., themaximum outside air wet bulb temperature at which free cooling isthermodynamically viable), and M_(big) is a sufficiently large number(e.g., M_(big)=10¹⁰).

In both the binary penalty approach and the continuous penalty approach,state transition controller 910 can use mixed integer linear programmingto optimize the total cost J_(total) over the duration of the horizon.For example, consider a twelve hour horizon with a time step each hour(i.e., k=1 . . . 12). As a result of the optimization, state transitioncontroller 910 can generate the following vectors X₁, X₂, and X₃ whichinclude the optimal values of X_(1,k), X_(2,k), and X_(3,k) at each ofthe twelve time steps:

X ₁=[0,0,0,Q ₄ ,Q ₅ ,Q ₆ ,Q ₇ ,Q ₈,0,Q ₁₀ ,Q ₁₁,0]

X ₂=[Q ₁ ,Q ₂ ,Q ₃,0,0,0,0,0,0,0,0,Q ₁₂]

X ₃=[0,0,0,0,0,0,0,0,Q ₉,0,0,0]

In this example, the cooling load was allocated to free cooling duringtime steps 4-8 and 10-11, as indicated by the non-zero values Q₄, Q₅,Q₆, Q₇, Q₈, Q₁₀, and Q₁₁ in vector X₁. This indicates that free coolingis economically optimal during hours 4-8 and 10-11 of the horizon. Thecooling load was allocated to mechanical cooling during time steps 1-3and 12, as indicated by the non-zero values of Q₁, Q₂, Q₃, and Q₁₂ invector X₂. This indicates that mechanical cooling is economicallyoptimal during hours 1-3 and 12 of the horizon. The cooling load wasallocated to neither free cooling nor mechanical cooling during timestep 9, as indicated by the value of Q₉ in vector X₃. This indicatesthat neither free cooling nor mechanical cooling is economically optimalduring hour 9 of the horizon, and the cooling load is shifted to thermalenergy storage or another subplant represented by X₃.

Each of state controllers 920 can operate as a closed loop controllerwithin the corresponding operating state 1002-1006. For example,mechanical cooling state controller 922 can control system operation inmechanical cooling state 1002, free cooling state controller 924 cancontrol system operation in free cooling state 1004, and no coolingstate controller 926 can control system operation in no cooling state1006. In some embodiments, each of state controllers 920 becomes activein response to a determination that state transition controller 910 hastransitioned into the corresponding operating state and inactive inresponse to a determination that state transition controller 910 hastransitioned out of the corresponding operating state. For example,mechanical cooling state controller 922 can become active in response toa determination that state transition controller 910 has transitionedinto mechanical cooling state 1002 and inactive in response to adetermination that state transition controller 910 has transitioned outof mechanical cooling state 1002. Similarly, free cooling statecontroller 924 can become active in response to a determination thatstate transition controller 910 has transitioned into free cooling state1004 and inactive in response to a determination that state transitioncontroller 910 has transitioned out of free cooling state 1004.

In some embodiments, each of state controllers 920 uses a differentcontrol algorithm, different control logic, and/or a different controlmethodology (e.g., PID control, extremum seeking control, modelpredictive control, etc.). This allows controller 640 to function asmultiple different controllers, each of which controls the operation ofHVAC system 600 in a designated operating state. For example, mechanicalcooling state controller 922 can control system operation in mechanicalcooling state 1002 by activating chiller 610 and using chiller 610 toprovide cooling for cooling load 608, as described with reference toFIG. 7. Free cooling state controller 924 can control system operationin free cooling state 1004 by deactivating chiller 610 and using coolingtower 602 to directly cool the chilled fluid in chilled fluid circuit636, as described with reference to FIG. 8.

Flow Diagram

Referring now to FIG. 11, a flow diagram of a process 1100 for operatinga HVAC system in a mechanical cooling state and a free cooling state isshown, according to an exemplary embodiment. Process 1100 can beperformed by one or more components of HVAC system 600, as describedwith reference to FIGS. 6-10. In some embodiments, process 1100 isperformed by controller 640.

Process 1100 is shown to include operating the HVAC system in amechanical cooling state (step 1102). In the mechanical cooling state(illustrated in FIG. 7), one or more chillers (e.g., chiller 610) canused to provide cooling for the chilled fluid in chilled fluid circuit636. Both chilled fluid circuit 636 and cooling tower circuit 632 can befluidly connected to chiller 610. Heat exchanger 606 may not be used andthe fluid conduits connecting to heat exchanger 606 may be blocked. Freecooling may not be used in the mechanical cooling state.

In the mechanical cooling state, controller 640 may operate valve 624 todirect the cool water from cooling tower 602 through condenser 612.Condenser 612 transfers heat from the refrigerant in refrigerationcircuit 634 to the cool water in cooling tower circuit 632, therebywarming the water. The warm water then flows from condenser 612 to valve626. Controller 640 operates valve 626 to direct the warm water tocooling tower 602. Cooling tower 602 transfers heat from the water tocooler air flowing through cooling tower 602. Controller 640 can operatefan 604 to modulate the airflow through cooling tower 602, which adjuststhe rate of heat transfer in cooling tower 602. Controller 640 can alsooperate pump 620 to modulate the flow rate of the water through coolingtower circuit 632, which adjusts the rate of heat transfer in coolingtower 602 and/or condenser 612.

In the mechanical cooling state, controller 640 may operate valve 630 todirect the fluid exiting cooling load 608 through evaporator 616.Evaporator 616 transfers heat from the fluid in chilled fluid circuit636 to the refrigerant in refrigeration circuit 634, thereby chillingthe fluid in chilled fluid circuit 636. The chilled fluid then flowsfrom evaporator 616 to valve 628. Controller 640 operates valve 628 todirect the chilled fluid to cooling load 608. Cooling load 608 rejectsheat to the chilled fluid, thereby providing cooling for cooling load608 and warming the chilled fluid. Controller 640 can operate pump 622to modulate the flowrate of the chilled fluid through chilled fluidcircuit 636, which adjusts the rate of heat transfer in evaporator 616and/or at cooling load 608.

Still referring to FIG. 11, process 1100 is shown to include predictingoutside air temperature {circumflex over (T)}_(OA) for each of aplurality of time steps in the future (step 1104). In some embodiments,the predicted outside air temperature {circumflex over (T)}_(OA) is awet bulb temperature of the air outside the building cooled by HVACsystem 600. The outside air temperature {circumflex over (T)}_(OA) canbe predicted using measurements from sensors 914 and/or weatherforecasts from weather service 916.

Process 1100 is shown to include identifying a free cooling temperaturethreshold T_(FC) (step 1106). In some embodiments, the free coolingtemperature threshold T_(FC) is a maximum outside air wet bulbtemperature at which free cooling is possible or economically viable.The free cooling temperature threshold T_(FC) can be based on thetemperature setpoint for the building or zone cooled by HVAC system 600.For example, the free cooling temperature threshold T_(FC) may beapproximately 40° F. for a building with a temperature setpoint around70° F.

Process 1100 is shown to include determining a minimum amount of freecooling time t_(min,FC) required for free cooling to be economicallyviable (step 1108). To make free cooling economically viable, the energyand cost savings achieved by free cooling should be sufficient toovercome the cost incurred as a result of transitioning betweenmechanical cooling state 1002 and free cooling state 1004. Step 1108 caninclude determining the minimum amount of time for which HVAC system 600must continue to operate in free cooling state 1004 in order to offsetthe cost incurred as a result of the state transition.

In some embodiments, step 1108 includes calculating the minimum freecooling time t_(min,FC). The minimum free cooling time t_(min,FC) can becalculated by weighing the free cooling energy savings against the costincurred as a result of switching from mechanical cooling state 1002 tofree cooling state 1004. For example, step 1108 can include using thefollowing equation to calculate the economic value of transitioning intofree cooling state 1004 and operating in free cooling state 1004:

Value_(FC) =Δt _(FC)Cost_(elec) P _(elec)−SwitchingPenalty

where Value_(FC) is the total economic value of transitioning into freecooling state 1004 and operating in free cooling state 1004 during thepredicted free cooling period, Δt_(FC) is the duration of the freecooling period (i.e., the predicted amount of time which will be spentin free cooling state 1004), Cost_(elec) is the estimated per unit costof electricity during the free cooling period

$( {{e.g.},\frac{\$}{kWh}} ),$

P_(elec) is the estimated free cooling energy savings per unit timeduring the free cooling period (e.g., kW), and Switching Penalty is theeconomic or monetary cost (e.g., $) incurred as a result of switchingfrom mechanical cooling state 1002 to free cooling state 1004.

In the previous equation, the term Δt_(FC)Cost_(elec)P_(elec) representsthe cost savings resulting from the use of free cooling relative tomechanical cooling over the duration of the free cooling period. Forexample, the product of energy cost Cost_(elec)

$( {{e.g.},\frac{\$}{kWh}} )$

and energy savings per unit time P_(elec) (e.g., kW) represents theeconomic cost of electricity which is saved by the use of free coolingduring each time step of the free cooling period

$( {{e.g.},\frac{\$}{hour}} ).$

Multiplying this savings per unit time by the duration of the freecooling period Δt_(FC) (e.g., hours) results in the total cost savingsover the duration of the free cooling period. The term SwitchingPenaltyrepresents the economic cost incurred as a result of the statetransition. As previously described, the economic cost may result fromincreased electricity consumption during chiller start-up or shut-down,increased equipment degradation resulting from switching chiller 610on/off, inefficient chiller operation while chiller 610 is starting-upor shutting-down, electricity required to operate valves 624-630, and/orany other economic costs which are incurred as a result of the statetransition.

Step 1108 can include calculating the minimum free cooling time byfinding the duration of the free cooling period Δt_(FC) which results ina total economic value of zero (i.e., Value=0). For example, the minimumfree cooling time t_(min,FC) can be calculated using the followingequation:

0 = t_(min , FC)Cost_(elec)P_(elec) − SwitchingPenalty$t_{\min,{FC}} = \frac{{Switching}\; {Penalty}}{{Cost}_{elec}P_{elec}}$

where the values of Cost_(elec), P_(elec), and SwitchingPenalty haveknown values. The value of SwitchingPenalty can be fixed, whereas thevalue of Cost_(elec) can be received from an energy utility or predictedbased upon past costs of electricity. The value of P_(elec) can bepredicted or estimated based on the amount of cooling required bycooling load 608.

Still referring to FIG. 11, process 1100 is shown to include determiningwhether the predicted outside air temperature {circumflex over (T)}_(OA)will be less than or equal to the free cooling temperature thresholdT_(FC) for a duration greater than or equal to the minimum free coolingtime (step 1110). If the predicted outside air temperature {circumflexover (T)}_(OA) will not remain below the temperature threshold T_(FC)for at least the minimum free cooling time t_(min,FC) (i.e., the resultof step 1110 is “no”), process 1100 may return to step 1102 and continueto operate HVAC system 600 in the mechanical cooling state 1002.However, if the predicted outside air temperature {circumflex over(T)}_(OA) will remain below the temperature threshold T_(FC) for atleast the minimum free cooling time (i.e., the result of step 1110 is“yes”), process 1100 may proceed to step 1112.

Process 1100 is shown to include operating the HVAC system in a freecooling state (step 1112). In the free cooling state, heat exchanger 606can be used to provide cooling for the chilled fluid in chilled fluidcircuit 636. Both chilled fluid circuit 636 and cooling tower circuit632 can be fluidly connected to heat exchanger 606. Chiller 610 may notbe used in the free cooling state and the fluid conduits connecting tochiller 610 can be blocked.

In the free cooling state, controller 640 can operate valve 624 todirect the cool water from cooling tower 602 through heat exchanger 606.Heat exchanger 606 transfers heat from the fluid in chilled fluidcircuit to the cool water in cooling tower circuit 632, thereby warmingthe water. The warm water then flows from heat exchanger 606 to valve626. Controller 640 operates valve 626 to direct the warm water tocooling tower 602. Cooling tower 602 transfers heat from the water tocooler air flowing through cooling tower 602. Controller 640 can operatefan 604 to increase or decrease the airflow through cooling tower 602,which increases or decreases the rate of heat transfer in cooling tower602. Controller 640 can also operate pump 620 to modulate the flow rateof the water through cooling tower circuit 632, which adjusts the rateof heat transfer in cooling tower 602 and/or heat exchanger 606.

In the free cooling state, controller 640 can operate valve 630 todirect the fluid exiting cooling load 608 through heat exchanger 606.Heat exchanger 606 transfers heat from the fluid in chilled fluidcircuit 636 to the water in cooling tower circuit 632, thereby chillingthe fluid in chilled fluid circuit 636. The chilled fluid then flowsfrom heat exchanger 606 to valve 628. Controller 640 operates valve 628to direct the chilled fluid to cooling load 608. Cooling load 608rejects heat to the chilled fluid, thereby providing cooling for coolingload 608 and warming the chilled fluid. Controller 640 can operate pump622 to modulate the flowrate of the chilled fluid through chilled fluidcircuit 636, which adjusts the rate of heat transfer in heat exchanger606 and/or at cooling load 608.

Still referring to FIG. 11, process 1100 is shown to include determininga minimum amount of mechanical cooling time t_(min,FC) required tojustify mechanical cooling (step 1114) and determining whether thepredicted outside air temperature {circumflex over (T)}_(OA) will begreater than or equal to the free cooling temperature threshold T_(FC)for a duration greater than or equal to the minimum mechanical coolingtime t_(min,FC) (step 1116). If the predicted outside air temperature{circumflex over (T)}_(OA) will not remain above the temperaturethreshold T_(FC) for at least the minimum mechanical cooling timet_(min,FC) (i.e., the result of step 1116 is “no”), process 1100 mayreturn to step 1112 and continue to operate HVAC system 600 in the freecooling state 1004. However, if the predicted outside air temperature{circumflex over (T)}_(OA) will remain above the temperature thresholdT_(FC) for at least the minimum mechanical cooling time (i.e., theresult of step 1116 is “yes”), process 1100 may return to step 1102 andtransition HVAC system into the mechanical cooling state 1002.

Configuration of Exemplary Embodiments

The construction and arrangement of the systems and methods as shown inthe various exemplary embodiments are illustrative only. Although only afew embodiments have been described in detail in this disclosure, manymodifications are possible (e.g., variations in sizes, dimensions,structures, shapes and proportions of the various elements, values ofparameters, mounting arrangements, use of materials, colors,orientations, etc.). For example, the position of elements can bereversed or otherwise varied and the nature or number of discreteelements or positions can be altered or varied. Accordingly, all suchmodifications are intended to be included within the scope of thepresent disclosure. The order or sequence of any process or method stepscan be varied or re-sequenced according to alternative embodiments.Other substitutions, modifications, changes, and omissions can be madein the design, operating conditions and arrangement of the exemplaryembodiments without departing from the scope of the present disclosure.

The present disclosure contemplates methods, systems and programproducts on any machine-readable media for accomplishing variousoperations. The embodiments of the present disclosure can be implementedusing existing computer processors, or by a special purpose computerprocessor for an appropriate system, incorporated for this or anotherpurpose, or by a hardwired system. Embodiments within the scope of thepresent disclosure include program products comprising machine-readablemedia for carrying or having machine-executable instructions or datastructures stored thereon. Such machine-readable media can be anyavailable media that can be accessed by a general purpose or specialpurpose computer or other machine with a processor. By way of example,such machine-readable media can comprise RAM, ROM, EPROM, EEPROM, CD-ROMor other optical disk storage, magnetic disk storage or other magneticstorage devices, or any other medium which can be used to carry or storedesired program code in the form of machine-executable instructions ordata structures and which can be accessed by a general purpose orspecial purpose computer or other machine with a processor. Combinationsof the above are also included within the scope of machine-readablemedia. Machine-executable instructions include, for example,instructions and data which cause a general purpose computer, specialpurpose computer, or special purpose processing machines to perform acertain function or group of functions.

Although the figures show a specific order of method steps, the order ofthe steps may differ from what is depicted. Also two or more steps canbe performed concurrently or with partial concurrence. Such variationwill depend on the software and hardware systems chosen and on designerchoice. All such variations are within the scope of the disclosure.Likewise, software implementations could be accomplished with standardprogramming techniques with rule based logic and other logic toaccomplish the various connection steps, processing steps, comparisonsteps and decision steps.

What is claimed is:
 1. A control system comprising: equipment configuredto: produce a resource by executing a first process when operating in afirst state; and produce the resource by executing a second process whenoperating in a second state; and a controller configured to: calculate aminimum second state operating time based on an estimated cost savingsresulting from operating in the second state relative to operating inthe first state, wherein the minimum second state operating time is aminimum amount of time that the equipment is required to operate in thesecond state for the estimated cost savings to offset a cost oftransitioning into the second state; predict whether the second processwill be available for at least the minimum second state operating timeduring a plurality of future time steps; and transition the equipmentfrom operating in the first state to operating in the second state inresponse to a prediction that the second process will be available forat least the minimum second operating state time.
 2. The system of claim1, wherein the controller is configured to calculate the minimum secondstate operating time by: identifying the cost incurred as a result oftransitioning the equipment into the second state; estimating the costsavings resulting from operating the equipment to execute the secondprocess in the second state relative to operating the equipment toexecute the first process in the first state as a function of an amountof time the equipment operates in the second state; and determining theminimum amount of time the equipment is required to operate in thesecond state for the cost savings to be greater than or equal to thecost incurred.
 3. The system of claim 2, wherein the controller isconfigured to estimate the cost savings by: determining an amount ofenergy savings resulting from operating the equipment in the secondstate relative to operating the equipment in the first state;identifying a cost per unit energy for each of the future time stepsduring which the equipment will operate in the second; and calculatingthe cost savings by multiplying the amount of energy savings by the costper unit energy.
 4. The system of claim 2, wherein the cost incurredcomprises at least one of an economic cost of equipment degradation andan increase in electricity cost resulting from stopping and restartingthe equipment.
 5. The system of claim 1, wherein predicting whether thesecond process will be available for at least the minimum second stateoperating time during the plurality of future time steps comprises:predicting a value of a variable that indicates an availability of thesecond process during each of the plurality of future time steps; andfor each time step of the plurality of future time steps, determiningwhether the second process will be available during the time step bycomparing the value of the variable to a threshold value.
 6. The systemof claim 1, wherein the controller is configured to: predict whether thesecond process will be unavailable for at least a minimum first stateoperating time; and transition the equipment from operating in thesecond state to operating in the first state in response to a predictionthat the second process will be unavailable for at least the minimumfirst state operating time.
 7. The system of claim 6, wherein thecontroller is configured to: determine whether the second process isavailable at a current time; and transition the equipment from operatingin the second state to operating in a third state during which theresource is not produced in response to a determination that the secondprocess is unavailable at the current time.
 8. The system of claim 7,wherein the controller is configured to transition the equipment fromoperating in the second state to operating in the third state inresponse to a determination that the second process is unavailable atthe current time and predicted to become available within apredetermined amount of time.
 9. A controller for equipment, thecontroller comprising: one or more processors; and one or morenon-transitory computer-readable storage media communicably coupled tothe one or more processors and having instructions stored thereon that,when executed by the one or more processors, cause the one or moreprocessors to: operate the equipment to execute a first process in afirst state to produce a resource; operate the equipment to execute asecond process in a second state to produce the resource; calculate aminimum second state operating time based on an estimated cost savingsresulting from operating in the second state relative to operating inthe first state, wherein the minimum second state operating time is aminimum amount of time that the equipment is required to operate in thesecond state for the estimated cost savings to offset a cost oftransitioning into the second state; predict whether the second processwill be available for at least the minimum second state operating timeduring a plurality of future time steps; and transition the equipmentfrom operating in the first state to operating in the second state inresponse to a prediction that the second process will be available forat least the minimum second state operating time.
 10. The controller ofclaim 9, wherein the instructions cause the one or more processors tocalculate the minimum second state operating time by: identifying thecost incurred as a result of transitioning the equipment into the secondstate; estimating the cost savings resulting from operating theequipment in the second state relative to operating the equipment in thefirst state as a function of an amount of time the equipment operates inthe second state; and determining the minimum amount of time theequipment is required to operate in the second state for the costsavings to be greater than or equal to the cost incurred.
 11. Thecontroller of claim 10, wherein the instructions cause the one or moreprocessors to estimate the cost savings by: determining an amount ofenergy savings resulting from operating the equipment in the secondstate relative to operating the equipment in the first state;identifying a cost per unit energy for each of the future time stepsduring which the equipment will operate in the second state; andcalculating the cost savings by multiplying the amount of energy savingsby the cost per unit energy.
 12. The controller of claim 10, wherein thecost incurred comprises at least one of an economic cost of equipmentdegradation and an increase in electricity cost resulting from stoppingand restarting the equipment.
 13. The controller of claim 9, whereinpredicting whether the second process will be available for at least theminimum second state operating time during the plurality of future timesteps comprises: predicting a value of a variable that identifies anavailability of the second process for the plurality of future timesteps; and for each time step of the plurality of future time steps,determining whether the second process will be available during the timestep by comparing the value of the variable to a threshold value. 14.The controller of claim 9, wherein the instructions cause the one ormore processors to: predict whether the second process will beunavailable for at least a minimum first state operating time; andtransition the equipment from operating in second state to operating inthe first state in response to a prediction that the second process willbe unavailable for at least the minimum first state operating time. 15.The controller of claim 14, wherein the instructions cause the one ormore processors to: determine whether the second process is available ata current time; and transition the equipment from operating in thesecond state to operating in a third state during which the resource isnot produced in response to a determination that the second process isunavailable at the current time.
 16. A method for controlling equipment,the method comprising: producing a resource by operating equipment toexecute a first process when operating in a first state; and producingthe resource by operating equipment to execute a second process whenoperating equipment in a second state; calculating a minimum secondstate operating time based on an estimated cost savings resulting fromoperating in the second state relative to operating in the first state,wherein the minimum second state operating time is a minimum amount oftime that the equipment is required to operate in the second state forthe estimated cost savings to offset a cost of transitioning into thesecond state; predicting whether the second process will be availablefor at least the minimum second state operating time during a pluralityof future time steps; and transitioning the equipment from operating inthe first state to operating in the second state in response to aprediction that the second process will be available for at least theminimum second operating state time.
 17. The method of claim 16, whereincalculating the minimum second state operating time comprises:identifying the cost incurred as a result of transitioning the equipmentinto the second state; estimating the cost savings resulting fromoperating the equipment to execute the second process in the secondstate relative to operating the equipment to execute the first processin the first state as a function of an amount of time the equipmentoperates in the second state; and determining the minimum amount of timethe equipment is required to operate in the second state for the costsavings to be greater than or equal to the cost incurred.
 18. The methodof claim 17, wherein estimating the cost savings comprises: determiningan amount of energy savings resulting from operating the equipment inthe second state relative to operating the equipment in the first state;identifying a cost per unit energy for each of the future time stepsduring which the equipment will operate in the second; and calculatingthe cost savings by multiplying the amount of energy savings by the costper unit energy.
 19. The method of claim 16, wherein predicting whetherthe second process is available for at least the second state operatingtime during the plurality of future time steps comprises: predicting avalue of a variable that indicates an availability of the second processduring each of the plurality of future time steps; and for each timestep of the plurality of future time steps, determining whether thesecond process will be available during the time step by comparing thevalue of the variable to a threshold value.
 20. The method of claim 16,further comprising: predicting whether the second process will beunavailable for at least a minimum first state operating time; andtransitioning the equipment from operating in the second state tooperating in the first state in response to a prediction that the secondprocess will be unavailable for at least the minimum first stateoperating time.